I just want to say that in real life, obese kids are bullied by skinny kids. The "fat bully" stereotype is very harmful to society.
Edit: No, I know the obese kid being the bullied thing is not an absolute. One of my adoptive brothers was obese and took no shit from anyone. He was cool and kinda smart and also the class clown; and eventually he kinda thought he could get away with anything, bought into thug culture, and now he's in and out of jail.
Well the audience has to know who the good guys are, and what better way than by making the good guys look aesthetically pleasing compared to the bads?
That said when I was a kid, the bigger kids tended to be the bullies, I always assumed it was because they could hit harder.