As BlaatMekker also referred to this hint in 10.5: One possible cause of huge Time Machine backups, let's mention yet another tool to find large files on your disk: Disk Inventory X.

This program has nothing to do with Time Machine, but may help investigating problems, like when you wonder why your backup is a lot smaller than the space used on your hard disk. Note that this program will probably always report less total disk space in its window title than in its screen where one can select which disk to investigate (even when running as root and choosing menu View, Show Physical File Size). But if the reported total size is really a lot smaller than the actual totals used on your Mac, then it may help to run as root:

sudo "/Applications/Utilities/Disk Inventory X.app/Contents/MacOS/Disk Inventory X"