protected override object LoadPageStateFromPersistenceMedium()
{
    string viewState = Request.Form["__COMPRESSEDVS"];
    if (viewState != null)
    {
         byte[] data = Convert.FromBase64String(viewState);
         data = Utils.Decompress(data);
         LosFormatter lf = new LosFormatter();
         return lf.Deserialize(Convert.ToBase64String(data));
    }
    else
    {
         return base.LoadPageStateFromPersistenceMedium();
    }
}protected override void SavePageStateToPersistenceMedium(object viewState)
{
    if (Utils.IsMsAjaxCallback(Request))
    {
         LosFormatter lf = new LosFormatter();
         using (StringWriter writer = new StringWriter())
         {
             lf.Serialize(writer, viewState);
             string viewStateString = writer.ToString();
             byte[] data = Convert.FromBase64String(viewStateString);
             data = Utils.Compress(data);
             ScriptManager.RegisterHiddenField(this, “__COMPRESSEDVS”, Convert.ToBase64String(data));
         }
    }
    else
    {
         base.SavePageStateToPersistenceMedium(viewState);
    }
}

=================

HERE IS THE Utils.cs CLASS

=================

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

public class Utils
{
    public static bool IsMsAjaxCallback(HttpRequest request)
    {
        return (request != null && request.Headers["X-MicrosoftAjax"] != null);
    }

    public static byte[] Compress(byte[] data)
    {
        using (MemoryStream ms = new MemoryStream())
        {
            using (GZipStream zip = new GZipStream(ms, CompressionMode.Compress, true))
            {
                zip.Write(data, 0, data.Length);
                zip.Dispose();
                return ms.ToArray();
            }
        }
    }

    public static byte[] Decompress(byte[] data)
    {
        using (MemoryStream ms = new MemoryStream())
        {
            int dataLength = BitConverter.ToInt32(data, 0);
            ms.Write(data, 0, data.Length);

            byte[] buffer = new byte[dataLength];

            ms.Position = 0;
            using (GZipStream zip = new GZipStream(ms, CompressionMode.Decompress))
            {
                zip.Read(buffer, 0, buffer.Length);
            }
            return buffer;
        }
    }
}