The word 'fat' isn't just a physical description that people 'choose' to be offended by, it has become an insult by some members of society, to mock those who are overweight and who (in their eyes) are socially undesirable. read more
I know people (men and women) who get mad when you say they're "not really that fat" or something too because they feel like you're lying to them, but if you agree with them when they call themselves fat, you're a jerk. read more