Grossly Huge VMWare Fusion Disk Image

[IronMac]

Having a bit of a problem with Fusion (latest version) in that the disk image is growing by leaps and bounds. I'm only using it to play Galactic Civilization II and occasionally surf using Firefox. But, it's grown to about 57 gigs.

First off, why is it growing so quickly and so large?
Second, how do I control it or shrink it because at the rate it's going it will consumer the whole HD in about six months!

Thanks!