Because we are told via various media such as magazines, tv etc that fat is ugly and repulsive, it's how we see ourselfs, we all like to think that we are slim good looking people like kate moss or david beckham when in reality we are not, so to target how we look when we feel insecure is an easy way to make someone feel bad, however, being overweight does not make you a bad person, and, as in the case of your second example, it deflects the fact that the person saying the hurtful thing is far from perfect.