Why does Time Machine back up way too much data and how can I slim this down? I just noticed that Time Machine is backing up 15 GB (it's been running for a few hours; this means that Time Machine thinks that 15 GB of data changed in one hour which is absurd). I've only been doing some programming in the Terminal, and the usual web browsing/iTunes etc.

Why is this happening? Is there a way to see which files does Time Machine think changed this much? How to stop this behavior?

Edit: It just occured to me that the two times it happened I was using a VMWare Virtual Machine. Is it possible that Apple made the terrible oversight of not having a working binary diff and it's backing up the Virtual Machine image in its entirety every time?
Yes, it is the whole virtual disk which gets saved, there is no binary diff.