OK, I've solved it on my machine, but I'm not sure if It will work for you as it sounds like you may have already tried what I did. Plus you're in a different OS. Anyway, this is exactly what I did, I'm not sure if all the steps are necessary, but its worth a shot.
1.Turn off time machine.
2. Unplug all of your external drives (except the one you are backing up onto).
3. Reformat the drive you intend to back up onto.
4. Go into your time machine preferences and click options.
5. Add all but one of your folders to the list of things to exclude from the backups - make sure the folder dosent contain very much, I picked one with about 1.5 gig of stuff. Its ok if its a subfolder.
6. Turn on time machine and tell it to make a backup. It could take as long as an hour for it to do this but it'll get there, dont be discoured if it sits there calculating for ages. When it finally starts to backup it will do the thing where it continues adding to the amount but it won't go on forever. Mine stopped about about 4 gig.
7. Reopen the time machine preferences and deselect some of the folders in the exceptions list. Try and do this in small blocks.
8. Tell time machine to perform another backup. It shouldn't take nearly as long as it did the first time.
9. Repeat steps 7 and 8 until you have backed up the whole drive.
By the time I finsihed the 384 gig of data I was backing up was only taking up about 400 gig of space.
Also I'd really appreciate it if someone would explain to me why the **** what I did worked. Just in case I get a simliar problem again in the future.
Hmm, interesting. Thanks for posting what worked for you. My problem actually randomly went away a couple weeks ago. I have no idea what I did to fix it, but TM is working as it should now. It must've been one of those many reformats/resets I did. If the problem ever comes up again I'll try the steps you posted.
. . .
Also I'd really appreciate it if someone would explain to me why the **** what I did worked.
Most likely, there's a directory problem on your internal HD that's causing the overestimate. Try verifying your internal HD, per #6 in Using Disk Utility.