Reduce "Metafile" memory usage?

I believe that the system cache releases its memory when required, but glitches can happen. Below are listed some possibilities.

Your 64-bits system allocates the video memory in the first 4GB of memory. So if your video card has lots of memory, that memory is lost, as described in :
The usable memory may be less than the installed memory on Windows 7-based computers

You may examine the memory usage on the computer by opening the Device Manager and under the View menu, choose "Resources by connection". Expand the Memory node and see if you can figure out what, if anything, is occupying the memory. In some cases, memory can be increased by disabling unused devices such as firewire.

You can also try and do the exact opposite of what is described in this article :
Increase The FileSystem Memory Cache Size In Windows 7

You can also try to Disable Windows 7 SuperFetch.

If nothing helps, it would be useful for you to post a screenshot of the Task Manager tab named Performance.