I would imagine that as with anything else there's a really small percentage of people who would react to insults by buckling down and getting obsessed with losing weight just to "show them." Seems like for most people it's just going to make them that much more self-conscious about going out to try to lose weight, like going to a gym, seeking out a nutritionist or personal trainer, looking for better food. I think it's hard for a lot of people to start what might be a long, hard process knowing that people are jeering at them and think they suck. (In fact, maybe the person has already lost a lot of weight but they're still overweight by someone else's standard. Instead of being rewarded for losing weight they're still getting insulted.)

Maybe they should compare it to other things: do people quit smoking because random people insult them? Get better grades in school? Is that an effective form of encouragement for most people in anything?