how come when i use a torrent downloading software,it occupies more GBs in

[simply_me]

my hard drive than its supposed to? i used azureus before and un-installed it coz when i tried to download an iso game, it hogged alot of gigs in my hard drive even if the game is like, 2-3 gigs only.so i uninstalled it without even finishing the download. so now i installed utorrent and ive started to download GTA san andreas iso, and the same problem happened, you see before i tried to download GTA, i did a disk defrag so my D drive has 85% free hard disk space, now when i started downloading GTA, (i chose to download it with flashget insted, the latest version now suppports torrents) and like 3 days later i did a defrag, when i had my drive D analyzed, it now only has 72% free space? GTA is only 3.95 GB big, so i have to have at least 81 GB left of free space. where did that 9GB additional space come from?? and i should now, coz i didnt install any software aside from utorrent and the GTA unfinished iso. the same thing happened when i was using azureus so i uninstalled it. i only have 40GB hard disk space.