Time machine wants to backup too much

Question

Time Machine stopped this morning, stating it has too little disk space left. It wants to back up 200 gigs, but my local hard drive has 240 gig used and 150 gig are excluded in the Time Machine options, so this leaves 90 gigs of data at most that can be backed up, and I am pretty sure only very few files have changed (no big update or similar). So I would estimate a backup of, at most, 1 gig.

How can this happen? Any idea?