Fat people - girls especially - are an easy target. Also, you can insult someone about their weight and it's totally accepted socially, in a way that commenting on their race, colour or sexuality wouldn't be.
I also think that fat girls tend to get insulted because most people think they wouldn't 'fight back', either verbally or physically. I'm not the thinnest person in the world, and I've been told by my friends that sometimes I look like an easy target ... I look way too nice/shy to turn round and take someone on. That's probably true - I got insulted by the girl behind the counter in Starbucks the other day. I asked for a shot of syrup in my coffee and she handed it to me with the comment 'extra syrup for the extra fat'. (Ok, shouldn't be drinking Starbucks or flavoured coffee but come on ...) However, I didn't say anything because I know that as a larger person I'd be fighting a loosing battle, as most people don't see anything wrong with that kind of behaviour.