What makes IIS7 W3WP Process use massive amounts of memory?

We have a Windows 2003 (x32) Production Web Server (running IIS6, obviously) hosting a .NET 2.0 web services app and a published client download. The W3WP process uses approximately 150MB on average.

Our test machine is Windows 2008 x64 (running IIS7) hosting the same web services app and published client download. The W3WP process uses 750+ MB and often causes timeouts in the application.

What would cause W3WP to cache that much information (on the first call to the web services) and use so much memory?