First off, why do you care how much memory a process is taking up? All of that memory will be paged out to disk, and assuming it isn't being paged back in regularly then all it does is take up page file size which is usually irrelevant.
The WaIISHost process runs your role entry point code (OnStart, Run, StatusCheck, Changing, etc) and is typically implemented in WebRole.cs. If you want to reduce the memory size of this process then you can reduce the amount of memory being loaded by your role entry point code.
See http://blogs.msdn.com/b/kwill/archive/2011/05/05/windows-azure-role-architecture.aspx for more information about the WaIISHost.exe process and what it does.