
Personally I think it has to do with cognitive dissonance. The more effort someone puts into not being fat, and the more they put themselves through suffering to be really skinny or fit, the more they have to justify this behavior. They can usually justify it by reasoning that it being fat makes people ugly and miserable. If they see someone who is fat who they think is happy or even content with themselves, they have to hate them or insult them in order to be able to justify their own behavior.