Apple's magic Time Machine backup system works great. But it has one flaw: it's inefficient with large files which change frequently. It makes a whole new copy of the file each time, there's no diff mechanism. My Time Machine backups are 300 MB every hour despite me actually changing very little. The Time Tracker app helped me figure out what's going on.
Chrome stores your web history in one giant file per month: 100 MB in two weeks for me! That file is constantly changing. Arguably history should be backed up, it's valuable user data, but I'd rather not if it's this inefficient. Unfortunately there's no easy way to exclude just the history database; the filename changes every month and the directory it is in has other useful stuff.
My entire Dropbox repository is being backed up whether I change anything or not. I'm not the first to see this problem but it's not clear what's going on. (My guess: v1.1.40 and Lion.) My solution was to simply stop backing up Dropbox (why bother?). A related issue is Dropbox maintains databases in .dropbox that can also get unwieldy.
Under the hood Time Machine is pretty simple; it's mostly doing the same hard link trick that rsnapshot uses with some help finding changed files from the filesystem. The GUI and easy setup is the magic, the sort of product Apple is uniquely good at building.
See also About Time Machine