A new feature to ASP.NET 4.0 is session state compression when you use SQL Server or the ASP.NET State Service. These are the two options available for ASP.NET in a web farm. Because both options involve storing state information outside a web application's worker process, session state has to be serialized before it is sent to remote storage. Depending on how much information a developer saves in session state, the size of the serialized data can grow quite large.
ASP.NET 4.0 introduces a new compression option for both kinds of out-of-process session-state providers. ASP.NET will compress and decompress serialized session state.
To enable this update the sessionState in the web.config file:
Initial Catalog=aspnetstate;User ID=user;Password=password;"