using System;
using System.IO;
using System.IO.Compression;
using System.Globalization;
using System.Web;


public class JsonCompressionModule : IHttpModule
{
   public JsonCompressionModule()
   {
   }

   public void Dispose()
   {
   }

   public void Init(HttpApplication app)
   {
       app.PreRequestHandlerExecute += new EventHandler(Compress);
   }

   private void Compress(object sender, EventArgs e)
   {
       HttpApplication app = (HttpApplication)sender;
       HttpRequest request = app.Request;
       HttpResponse response = app.Response;
     
       if (request.ContentType.ToLower(CultureInfo.InvariantCulture).StartsWith("application/json"))
       {
           if (!((request.Browser.IsBrowser("IE")) && (request.Browser.MajorVersion <= 6)))
           {
               string acceptEncoding = request.Headers["Accept-Encoding"];

               if (!string.IsNullOrEmpty(acceptEncoding))
               {
                   acceptEncoding = acceptEncoding.ToLower(CultureInfo.InvariantCulture);

                   if (acceptEncoding.Contains("gzip"))
                   {
                       response.Filter = new GZipStream(response.Filter, CompressionMode.Compress);
                       response.AddHeader("Content-encoding", "gzip");
                   }
                   else if (acceptEncoding.Contains("deflate"))
                   {
                       response.Filter = new DeflateStream(response.Filter, CompressionMode.Compress);
                       response.AddHeader("Content-encoding", "deflate");
                   }
               }
           }
       }
   }
}

if ( !request.Url.PathAndQuery.ToLower().Contains( ".asmx" ) )
   return;

<httpModules>    
   <add name="JsonCompressionModule" type="JsonCompressionModule"/>
</httpModules>