Help New SD CARD almost full


I just bought a new Desire HD last week, lovin it so far!, but i noticed that my 8 GB sd card is already over half full. phone says i have only 2.97 GB left. i've only downloaded ~15 apps so far (none are very big) so i was wondering how this could be. i'm new to android but i don't think the OS is that big. is it? any thoughts are appreciated