diff --git a/Facepunch.Steamworks/BaseSteamworks.cs b/Facepunch.Steamworks/BaseSteamworks.cs
index 76803fb..ee06870 100644
--- a/Facepunch.Steamworks/BaseSteamworks.cs
+++ b/Facepunch.Steamworks/BaseSteamworks.cs
@@ -72,10 +72,15 @@ public enum MessageType : int
         /// <summary>
         /// Global callback type
         /// </summary>
+        internal void AddCallback<T, TSmall>( Action<T> Callback, int id )
+        {
+            var callback = new Callback<T, TSmall>( IsGameServer, id, Callback );
+            Disposables.Add( callback );
+        }
+
         internal void AddCallback<T>( Action<T> Callback, int id )
         {
-            var callback = new Callback<T>( IsGameServer, id, Callback );
-            Disposables.Add( callback );
+            AddCallback<T, T>( Callback, id );
         }
 
         public Action OnUpdate;
diff --git a/Facepunch.Steamworks/Callbacks/Networking.cs b/Facepunch.Steamworks/Callbacks/Networking.cs
index 1f7add2..dd77ffd 100644
--- a/Facepunch.Steamworks/Callbacks/Networking.cs
+++ b/Facepunch.Steamworks/Callbacks/Networking.cs
@@ -6,7 +6,7 @@
 
 namespace Facepunch.Steamworks.Callbacks.Networking
 {
-    [StructLayout( LayoutKind.Sequential )]
+    [StructLayout( LayoutKind.Sequential, Pack = 1 )]
     internal class P2PSessionRequest
     {
         public ulong SteamID;
@@ -14,7 +14,7 @@ internal class P2PSessionRequest
         public const int CallbackId = Index.Networking + 2;
     };
 
-    [StructLayout( LayoutKind.Sequential )]
+    [StructLayout( LayoutKind.Sequential, Pack = 1 )]
     internal class P2PSessionConnectFail
     {
         public ulong SteamID;
diff --git a/Facepunch.Steamworks/Callbacks/User.cs b/Facepunch.Steamworks/Callbacks/User.cs
index b988c2d..2838f99 100644
--- a/Facepunch.Steamworks/Callbacks/User.cs
+++ b/Facepunch.Steamworks/Callbacks/User.cs
@@ -6,9 +6,15 @@
 
 namespace Facepunch.Steamworks.Callbacks.User
 {
-    [StructLayout( LayoutKind.Explicit )]
+    [StructLayout( LayoutKind.Sequential, Pack = 1 )]
     internal struct ValidateAuthTicketResponse
     {
+        public ulong SteamID;
+        public int AuthResponse;
+        public ulong OwnerSteamID;
+
+        public const int CallbackId = Index.User + 43;
+
         public enum Response : int
         {
             Okay = 0,                         // Steam has verified the user is online, the ticket is valid and ticket has not been reused.
@@ -22,15 +28,6 @@ public enum Response : int
             AuthTicketInvalid = 8,            // This ticket is not from a user instance currently connected to steam.
             PublisherIssuedBan = 9,           // The user is banned for this game. The ban came via the web api and not VAC
         };
-
-        [FieldOffset(0)]
-        public ulong SteamID;
-        [FieldOffset(8)]
-        public Response AuthResponse;
-        [FieldOffset(12)]
-        public ulong OwnerSteamID;
-
-        public const int CallbackId = Index.User + 43;
     };
 
 
diff --git a/Facepunch.Steamworks/Callbacks/Workshop.cs b/Facepunch.Steamworks/Callbacks/Workshop.cs
index c6a8a44..9d09270 100644
--- a/Facepunch.Steamworks/Callbacks/Workshop.cs
+++ b/Facepunch.Steamworks/Callbacks/Workshop.cs
@@ -4,70 +4,104 @@
 namespace Facepunch.Steamworks.Callbacks.Workshop
 {
 
-    [StructLayout( LayoutKind.Explicit )]
-    internal class ItemInstalled
+    [StructLayout( LayoutKind.Sequential, Pack = 8 )]
+    internal struct ItemInstalled
     {
-        [FieldOffset(0)]
         public uint AppId;
-        [FieldOffset(4)]
         public ulong FileId;
 
         public const int CallbackId = Index.UGC + 5;
+
+        [StructLayout( LayoutKind.Sequential, Pack = 4 )]
+        internal struct Small
+        {
+            public uint AppId;
+            public ulong FileId;
+        };
     };
 
-    [StructLayout( LayoutKind.Explicit )]
-    internal class DownloadResult
+    [StructLayout( LayoutKind.Sequential, Pack = 8 )]
+    internal struct DownloadResult
     {
-        [FieldOffset(0)]
         public uint AppId;
-        [FieldOffset(4)]
         public ulong FileId;
-        [FieldOffset(12)]
         public Result Result;
 
         public const int CallbackId = Index.UGC + 6;
+
+        [StructLayout( LayoutKind.Sequential, Pack = 4 )]
+        internal struct Small
+        {
+            public uint AppId;
+            public ulong FileId;
+            public Result Result;
+        };
     };
 
-    internal class QueryCompleted : CallResult<QueryCompleted.Data>
+
+    internal class QueryCompleted : CallResult<QueryCompleted.Data, QueryCompleted.Data.Small>
     {
         public override int CallbackId { get { return Index.UGC + 1; } }
 
-        [StructLayout( LayoutKind.Sequential )]
+        [StructLayout( LayoutKind.Sequential, Pack = 8 )]
         internal struct Data
         {
             internal ulong Handle;
             internal int Result;
-            internal uint m_unNumResultsReturned;
-            internal uint m_unTotalMatchingResults;
-            [MarshalAs(UnmanagedType.I1)]
-            internal bool m_bCachedData; // indicates whether this data was retrieved from the local on-disk cache
+            internal uint NumResultsReturned;
+            internal uint TotalMatchingResults;
+            internal bool CachedData;
+
+            [StructLayout( LayoutKind.Sequential, Pack = 4 )]
+            internal struct Small
+            {
+                internal ulong Handle;
+                internal int Result;
+                internal uint NumResultsReturned;
+                internal uint TotalMatchingResults;
+                internal bool CachedData;
+            };
         };
     }
 
-    internal class CreateItem : CallResult<CreateItem.Data>
+    internal class CreateItem : CallResult<CreateItem.Data, CreateItem.Data.Small>
     {
         public override int CallbackId { get { return Index.UGC + 3; } }
 
-        [StructLayout( LayoutKind.Sequential )]
+        [StructLayout( LayoutKind.Sequential, Pack = 8 )]
         internal struct Data
         {
             internal Result Result;
             internal ulong FileId;
-            [MarshalAs(UnmanagedType.I1)]
-            internal bool NeedsLegalAgreement; 
+            internal bool NeedsLegalAgreement;
+
+            [StructLayout( LayoutKind.Sequential, Pack = 4 )]
+            internal struct Small
+            {
+                internal Result Result;
+                internal ulong FileId;
+ 
+                internal bool NeedsLegalAgreement;
+            };
         };
     }
 
-    internal class SubmitItemUpdate : CallResult<SubmitItemUpdate.Data>
+    internal class SubmitItemUpdate : CallResult<SubmitItemUpdate.Data, SubmitItemUpdate.Data.Small>
     {
         public override int CallbackId { get { return Index.UGC + 4; } }
 
-        [StructLayout( LayoutKind.Sequential )]
+        [StructLayout( LayoutKind.Sequential, Pack = 8 )]
         internal struct Data
         {
             internal Result Result;
-            [MarshalAs(UnmanagedType.I1)]
             internal bool NeedsLegalAgreement;
+
+            [StructLayout( LayoutKind.Sequential, Pack = 4 )]
+            internal struct Small
+            {
+                internal Result Result;
+                internal bool NeedsLegalAgreement;
+            };
         };
     }
 }
diff --git a/Facepunch.Steamworks/Config.cs b/Facepunch.Steamworks/Config.cs
index 9f94f34..27d8392 100644
--- a/Facepunch.Steamworks/Config.cs
+++ b/Facepunch.Steamworks/Config.cs
@@ -18,6 +18,11 @@ public static class Config
         /// </summary>
         public static bool UseThisCall { get; set; } = true;
 
+        /// <summary>
+        /// Set this to true on Linux and OSX
+        /// </summary>
+        public static bool PackSmall { get; set; } = false;
+
 
         /// <summary>
         /// The Native dll to look for. This is the steam_api.dll renamed.
diff --git a/Facepunch.Steamworks/Interfaces/Workshop.Item.cs b/Facepunch.Steamworks/Interfaces/Workshop.Item.cs
index 96eaaf7..58e01da 100644
--- a/Facepunch.Steamworks/Interfaces/Workshop.Item.cs
+++ b/Facepunch.Steamworks/Interfaces/Workshop.Item.cs
@@ -65,6 +65,7 @@ public void Download( bool highPriority = true )
 
             private void OnFileDownloaded( ulong fileid, Callbacks.Result result )
             {
+                if ( fileid != Id ) return;
 
                 workshop.OnFileDownloaded -= OnFileDownloaded;
                 UpdateState();
@@ -75,6 +76,7 @@ private void OnFileDownloaded( ulong fileid, Callbacks.Result result )
 
             private void OnItemInstalled( ulong fileid )
             {
+                if ( fileid != Id ) return;
 
                 workshop.OnItemInstalled -= OnItemInstalled;
                 UpdateState();
diff --git a/Facepunch.Steamworks/Interfaces/Workshop.Query.cs b/Facepunch.Steamworks/Interfaces/Workshop.Query.cs
index dd453a2..835b8c3 100644
--- a/Facepunch.Steamworks/Interfaces/Workshop.Query.cs
+++ b/Facepunch.Steamworks/Interfaces/Workshop.Query.cs
@@ -86,8 +86,8 @@ public unsafe void Run()
 
             void OnResult( QueryCompleted.Data data )
             {
-                Items = new Item[data.m_unNumResultsReturned];
-                for ( int i = 0; i < data.m_unNumResultsReturned; i++ )
+                Items = new Item[data.NumResultsReturned];
+                for ( int i = 0; i < data.NumResultsReturned; i++ )
                 {
                     SteamUGCDetails_t details = new SteamUGCDetails_t();
                     workshop.ugc.GetQueryUGCResult( data.Handle, (uint)i, ref details );
@@ -95,7 +95,7 @@ void OnResult( QueryCompleted.Data data )
                     Items[i] = Item.From( details, workshop );
                 }
 
-                TotalResults = (int)data.m_unTotalMatchingResults;
+                TotalResults = (int)data.TotalMatchingResults;
 
                 Callback.Dispose();
                 Callback = null;
diff --git a/Facepunch.Steamworks/Interfaces/Workshop.cs b/Facepunch.Steamworks/Interfaces/Workshop.cs
index af57e54..a99dc61 100644
--- a/Facepunch.Steamworks/Interfaces/Workshop.cs
+++ b/Facepunch.Steamworks/Interfaces/Workshop.cs
@@ -22,8 +22,8 @@ internal Workshop( BaseSteamworks steamworks, ISteamUGC ugc, ISteamRemoteStorage
             this.steamworks = steamworks;
             this.remoteStorage = remoteStorage;
 
-            steamworks.AddCallback<DownloadResult>( onDownloadResult, DownloadResult.CallbackId );
-            steamworks.AddCallback<ItemInstalled>( onItemInstalled, ItemInstalled.CallbackId );
+            steamworks.AddCallback<DownloadResult, DownloadResult.Small>( onDownloadResult, DownloadResult.CallbackId );
+            steamworks.AddCallback<ItemInstalled, DownloadResult.Small>( onItemInstalled, ItemInstalled.CallbackId );
         }
 
         public void Dispose()
diff --git a/Facepunch.Steamworks/Interop/CallResult.cs b/Facepunch.Steamworks/Interop/CallResult.cs
index 73bc612..bf40610 100644
--- a/Facepunch.Steamworks/Interop/CallResult.cs
+++ b/Facepunch.Steamworks/Interop/CallResult.cs
@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Reflection;
 using System.Runtime.InteropServices;
 using System.Text;
 using Valve.Steamworks;
@@ -19,14 +20,19 @@ public void Dispose()
         internal abstract void Run( ISteamUtils utils );
     }
 
-    internal unsafe abstract class CallResult<T> : CallResult
+    internal unsafe abstract class CallResult<T, TSmall> : CallResult where T: new()
     {
+        public static FieldInfo[] SourceFields = typeof( TSmall ).GetFields();
+        public static FieldInfo[] DestFields = typeof( T ).GetFields();
+
         public abstract int CallbackId { get; }
-        public Action<T> OnResult;
+        public Action<T> OnResult;        
 
         internal override void Run( ISteamUtils utils )
         {
-            var datasize = Marshal.SizeOf( typeof( T ) );
+            var packSmall = Config.PackSmall;
+
+            var datasize = packSmall ? Marshal.SizeOf( typeof( TSmall ) ) :  Marshal.SizeOf( typeof( T ) );
             var data = stackalloc byte[ datasize ];
             bool failed = false;
 
@@ -36,10 +42,28 @@ internal override void Run( ISteamUtils utils )
                 return;
             }
 
-            var dataObject = (T)Marshal.PtrToStructure( (IntPtr) data, typeof( T ) );
+            if ( packSmall )
+            {
+                var dataTarget = new T();
+                var dataObject = (TSmall)Marshal.PtrToStructure( (IntPtr) data, typeof( TSmall ) );
+
+                for ( int i=0; i<SourceFields.Length; i++ )
+                {
+                    DestFields[i].SetValue( dataTarget, SourceFields[i].GetValue( dataObject ) );
+                }
+
+                if ( OnResult != null )
+                    OnResult( dataTarget );
+            }
+            else
+            {
+                var dataObject = (T)Marshal.PtrToStructure( (IntPtr) data, typeof( T ) );
+
+                if ( OnResult != null )
+                    OnResult( dataObject );
+            }
+
 
-            if ( OnResult != null )
-                OnResult( dataObject );
         }        
     }
 }
\ No newline at end of file
diff --git a/Facepunch.Steamworks/Interop/Callback.This.cs b/Facepunch.Steamworks/Interop/Callback.This.cs
index b108d35..9e998bf 100644
--- a/Facepunch.Steamworks/Interop/Callback.This.cs
+++ b/Facepunch.Steamworks/Interop/Callback.This.cs
@@ -9,37 +9,28 @@ namespace Facepunch.Steamworks.Interop.VTable.This
     [StructLayout( LayoutKind.Sequential )]
     internal struct Callback
     {
-        [UnmanagedFunctionPointer( CallingConvention.ThisCall )]
-        public delegate void Result( IntPtr thisptr, IntPtr pvParam );
+        [UnmanagedFunctionPointer( CallingConvention.ThisCall )]    public delegate void Result( IntPtr thisptr, IntPtr pvParam );
+        [UnmanagedFunctionPointer( CallingConvention.ThisCall )]    public delegate int GetSize( IntPtr thisptr );
 
-        [UnmanagedFunctionPointer( CallingConvention.ThisCall )]
-        public delegate int GetSize( IntPtr thisptr );
+        [MarshalAs(UnmanagedType.FunctionPtr)]  public Result RunCallResult;
+        [MarshalAs(UnmanagedType.FunctionPtr)]  public Result RunCallback;
+        [MarshalAs(UnmanagedType.FunctionPtr)]  public GetSize GetCallbackSizeBytes;
 
-        [MarshalAs(UnmanagedType.FunctionPtr)]
-        public Result m_RunCallback;
-
-        [MarshalAs(UnmanagedType.FunctionPtr)]
-        public Result m_RunCallResult;
-
-        [MarshalAs(UnmanagedType.FunctionPtr)]
-        public GetSize m_GetCallbackSizeBytes;
-
-        internal static IntPtr Get( Action<IntPtr> onRunCallback, Func<int> getSize, Interop.Callback cb )
+        internal static IntPtr Get( Action<IntPtr, IntPtr> onRunCallback, int size, Interop.Callback cb )
         {
-            var size = Marshal.SizeOf( typeof( Callback ) );
-            var ptr = Marshal.AllocHGlobal( size );
+            var ptr = Marshal.AllocHGlobal( Marshal.SizeOf( typeof( Callback ) ) );
 
-            Callback.Result da = ( _, p ) => onRunCallback( p );
-            Callback.GetSize dc = ( _ ) => getSize();
+            Callback.Result da = ( _, p ) => {  onRunCallback( _, p ); Console.WriteLine( "Callback.Result: {0}", _.ToInt64() ); };
+            Callback.GetSize dc = ( _ ) => { return size; };
 
             cb.AddHandle( GCHandle.Alloc( da ) );
             cb.AddHandle( GCHandle.Alloc( dc ) );
 
             var table = new Callback()
             {
-                m_RunCallResult = da,
-                m_RunCallback = da,
-                m_GetCallbackSizeBytes = dc
+                RunCallback = da,
+                RunCallResult = da,
+                GetCallbackSizeBytes = dc
             };
 
             Marshal.StructureToPtr( table, ptr, false );
diff --git a/Facepunch.Steamworks/Interop/Callback.cs b/Facepunch.Steamworks/Interop/Callback.cs
index c6667f8..e20ecb5 100644
--- a/Facepunch.Steamworks/Interop/Callback.cs
+++ b/Facepunch.Steamworks/Interop/Callback.cs
@@ -48,7 +48,7 @@ internal void AddHandle( GCHandle gCHandle )
         }
     }
 
-    internal partial class Callback<T> : Callback
+    internal partial class Callback<T, TSmall> : Callback
     {
         public int CallbackId = 0;
         public bool GameServer = false;
@@ -87,10 +87,13 @@ public override void Dispose()
             base.Dispose();
         }
 
-        private void OnRunCallback( IntPtr ptr )
+        private void OnRunCallback( IntPtr thisObject, IntPtr ptr )
         {
             if ( callbackPin == null ) throw new System.Exception( "Callback wasn't pinned!" );
             if ( vTablePtr == IntPtr.Zero ) throw new System.Exception( "vTablePtr wasn't pinned!" );
+            if ( thisObject != IntPtr.Zero && thisObject != callbackPin.AddrOfPinnedObject() ) throw new System.Exception( "This wasn't valid!" );
+
+            if ( Config.PackSmall && typeof(T) != typeof( TSmall ) ) throw new System.Exception( "Callback should use PackSmall" );
 
             T data = (T) Marshal.PtrToStructure( ptr, typeof(T) );
             Function( data );
@@ -121,7 +124,7 @@ void InitVTable()
         {
             if ( Config.UseThisCall )
             {
-                vTablePtr = VTable.This.Callback.Get( OnRunCallback, GetSize, this );
+                vTablePtr = VTable.This.Callback.Get( OnRunCallback, Marshal.SizeOf( typeof( T ) ), this );
                 return;
             }