when i restored my system xp 3 times the HDD became full?

hi
it happened when a problem has accourd with the system and i restored my system by the System Restore about 3 times and its become good but i noticed that the hard disk drive became seem full i returned it back the restore and the used size become larger .
i asked the IT person he said must be some hidden files but there’s none and he said your pc must be formatted
so plz give me some help.