Becoming fat is like an extremely localized natural disaster.
i.e. it happens because it's natural, and for the person involved, it's usually a disaster.
I really do think fat people who call people who're thinner than them 'fat' completely ridiculous, though. It's like, 'dude, no, you're not skinny either, you're just so big your reflection overlaps your mirror frame and you can't see your bulges DDD:'
But I've met a lot of nice fat people too, who don't think/act that way. So I reserve judgment until a fat person actively proves himself/herself to be an a**.