There is a great little 'prerelease' app from
http://www.charlessoft.com/ called "TimeTracker". What it does is scan your Time Machine backups and tell you exactly what was backed up. It's a great initial tool to troubleshooting your Time Machine activities.
Unfortunately, it doesn't work fully under Snow Leopard (it only reports how much was backed up, but not the exact files).
I followed the use of Time Machine since it came out -I'm not that technical, but I have noticed a lot of interesting behaviour.
For example, if you manually alter a file inside a wrapped-up folder such as the iPhoto Library, it will see that as a change and backup the ENTIRE library again (I was messing about with invisible files inside it a couple times). I think it doesn't do this any more, but who knows, since I don't really want to envoke it any further. Also, regarding iPhoto, if you so much as make a change when opening iPhoto (change an image, for example), it will recreate the iPhoto library database. This will envoke a backup up of the associated database files, which can be GBs, depending on your library size.
30GB is kind of a big backup, so it's worth checking out to see what you are changing. Keep in mind, the more 'non-default' options you use (third party add-ons, apps etc), then it's possible those options alter things that Apple have not accounted for (and to be honest, should be a responsibility of the third-party developer. Of course, I've seen default configurations do weird things in TIme Machine also, but it's easier to troubleshoot.
Good luck.
Edit:
You're in luck! With some stroke of fortune in timing, looks like Time Tracker was updated a week ago (first time I've seen since for nearly a year), and it's now working with my Snow Leopard time machine. You should easily be able to see what files are being backed up.