This might be obvious to most, but it's easy for novice VmWare users to forget.
When you take a snapshot, VmWare has to write down the full current status of your image, including RAM. If you take a snapshot while the image is running, VmWare will have to save the full RAM content, which might run up to several GBs. If you take the snapshot after your image has properly shut down, RAM content will simply be discarded, and VmWare will only have to deal with the actual disk.
TL;DR: Always shut down your image before snapshotting, and you'll save a lot of disk space.