Because they've managed to twist the world in their little fat heads into thinking that they are NORMAL, that being obese, even morbidly so, is normal for them, and some people. That's the dangers of fat acceptance, it teaches you that being morbidly obese is normal. Therefore, everyone in the world who is less than morbidly obese is thin, and people who are truly fit and in shape appear thin on an anorexic level. Since they pretty much never see an actually anorexic person out in the world, the only scale they have is this one: "I am morbidly obese, and that's normal. That guy is just obese, he's thinner. That woman is only a few pounds overweight, she must be damn thin! Oh my god, you can see that person's abs, they're so anorexic it's not natural!" And that's what fat acceptance and the shifting baselines of weight perception have done. I actually had to deal with this with my best friend and his family the other day. I have always been thin, but decided this summer that I just wanted to be more fit and started running (thank you Couch to 5k), and went to visit him and his family a week ago (none of them are fat, by the way). Now, I've only lost about 5lbs since I started running, but my body definitely has reorganized it's existing weight and I'm much more lean and muscled (especially in my legs) than I was before. Everyone in his family was immediately like "You've lost so much weight! Don't lose anymore, you'll be way to thin!" I am 5'4" and 118lbs. I could lose another 17lbs before I would be considered underweight, and yet they were already freaking out about me being too thin. People just no longer have the ability to recognize a healthy weight when everyone else is a fat fuck :(.