I have 16GB RAM in my Mac Mini; maxed out! I’ve been using all of it lately. I’ve even crashed the Mini because it used 100% of the RAM. Never done that before.
After some experimentation I’ve found that rebooting the Windows 7 virtual machine, running in VMWare Fusion 5.0.4, does not release RAM retained for File Cache. No idea why; I didn’t take the time to wait between reboots to see if the RAM would eventually be released.
The “fix” I’ve found is to purge the memory. Basically, it’s a manual release of the RAM allocated to File Cache.