The answer is simple. Fat people are different from around day zero. They're not just socially awkward, but also physically deficient. While their girth may be used to intimidate at an older age, as a youngster growing up they are the subject of jokes to the leaner, meaner kids. Pushed around, made fun of, an outcast, they have really one of two options as a defense. Either they will be become embittered towards the world, hating everyone and everything. They will think their intellect is what really seperates them from everyone else, that they are smarter than everyone else so they can all just suck one. Or, they will try to cross the bridge to make friends with the other kids. Humor is their weapon, and while it may not get others to stop picking on them, it will make them some other friends. They can band together with these other friends and ignore the onslaughts of the nasty children who hate what's different. Then, all the bullies do is give these chubby little tykes comediec fuel for their funny fires as adults.
This all may change, however, in America. You see, on average, the people are becoming more obese here, and rather than told to lose some weight the children are told it's not their fault. Rather than the one chubby kid in class, you get half a dozen or more. As more and more kids become fat from eating unhealthy food and told they don't need to excerise if they don't want to, skinny people will become the minority. Who knows, maybe then the thin ones will have to cope by being a funny man or an asshole. I doubt it, since the media will always employ only the most attractive and well-groomed to be the lead roles, with the fatso as the unhelpful side-kick. It may become more acceptable, but I doubt the thin kids of tommorrow will act like the fat kids of today. Maybe it'll just be middle of the road, fat kids will have less extreme personalities as while they may be excluded from sports, they will be excluded in mass amounts.