Imac G5 issues

I have a iMac g5 i use from time to time, i recently backed up data on it's hard drive freeing up 11 gigs. Today when i went on there was 1.24 gigs left. I havent downloaded anything and only use it for surfing the net. What could be causing this?

ps. i ran onyx but that only was able to free up a little space.