Yeah, that is something I have noticed. Most of the "superskinnies" could eat better, but their crimes are being skinnyfat at worst. Very few of them seem to be clearly and dangerously underweight. I think it is a combination of, as you say, a lack of underweight people in the sea of fatness and that many actually underweight people have genuine eating disorders and they obviously don't work with the show. I always figured the reason the show ended is because they simply ran out of thin people in the UK.