Because the forums are the busiest part of the site. If a thread has 20 replies, and you click on it, and it has 20 icons, there's automatically potentially 40 k of transfer... threads are clicked on tens of thousands of times a day, so if the file size was bigger, we'd run out of bandwidth more quickly.
If I'm thinking straight, your giraffe avatar is the equivelent of 640 letters/numbers/spaces in a post. Now really, how often do you see a post 640 letters/numbers/spaces long here? The avatars take up more bandwidth then posts!