Well, 200MB is probably the most it will use up. I've noticed this same occurrence, and I've got 1GB of RAM in my machine!
I am 99% sure it's the swap file for memory that's taking up this space... and I've never noticed it go above 200MB. 512Mb of RAM should be enough to run basic programs, although if you're keeping a LOT of programs open, that could be the reason your system is paging out memory to the disk.
The only way to recover this space, as far as I know, is to restart.