Why is it acceptable to make nasty comments about overweight people?

I have found that people readily make nasty comments about overweight people, whether they are somewhat overweight, very overweight or morbidly obese. This seems outrageous to me. In our society, people who make racist, sexist or otherwise bigoted remarks are, at best, looked down upon and, at worst, ostracized. However, it is perfectly acceptable for people to make mean comments about someone’s weight. Perhaps fluther can enlighten me about this.