In America, when s superficial man says that a overweight woman is not beautiful, what they really mean is that they would not have sex with her. That's it. Plain and simple. The reason why it is amn issue of sex is because in general, in this country, people who are overweight are trated as if they are not worthy of love or to be alive, because of their weight.
The norm has been to kick down at overweight people, but now there is a push for acceptance and inclusion the body shamers cannot handle. And as you said. being healthy is important; and being skinny does not mean that you are the picture of health.
Check this out, if you're interested.