Why is Time Machine over-stating the amount of data that needs to be backed up?
Time Machine uses FSEvents to determine what has changed, which is based on folders rather than files. Hence, I've always assumed† that the larger number indicates the total size of the folders it has to process. Next, within each folder it will evaluate each file. (In 2008, some wrote "Time Machine considers two files to be identical if their path and dates and size are the same", so some changes might even go unnoticed. But that's another story.)

If many files in a large folder have not changed, then the total number to actually back up is much lower, but Time Machine does not know until it has processed those. Note that files might have the extended attribute com.apple.metadata:com_apple_backup_excludeItem set, or might be excluded by Apple's factory settings, implying they won't be put on the backup. See Does Apple's Time Machine app really copy everthing on Super User.

(So: I guess you need to find large source folders that are small on the backup. Tools like Disk Inventory X or GrandPerspective could indeed show large folders, like in /private/var. Might be easy to compare that to the few folders on the backup?)



† Now that I think of it years later: seeing the size of my Downloads folder, I doubt that it's (always) showing the folder size in the amount of data it's going to back up. Downloading another small file does NOT make Time Machine claim it needs to back up the full size of that Downloads folder. (And yes, that folder is on my backups.)