Most people have different opinions about what is a healthy body, even health professionals.
Those who gripe about such things have their own cognitive distortion... they have ingrained behaviors to value physical attractiveness (and fitness) beyond any utility it might provide them.
Are you going to hate the fat girl's personality so much that you can't stand her? Probably not.
(Apologies for being crude) Is her pussy going to feel like sandpaper on your dick? Probably not.
You mistreat such people because you're afraid that if you were to become involved with them, your social status would decline. But the only reason humans care about that is because of mating prospects.
Your truths are subjective, and self-harming.
Which is why this notion that weight can only be criticized or celebrated in the context of beauty is nonsense. You can be both beautiful and deeply unhealthy. And that's what weight really is, a health issue. Hijacking genuine conversations about health and turning them into philosophical conversations about beauty is intellectually dishonest. It's easy to dismiss subjective opinions about beauty. It's difficult to address society-wide failures with weight management.
If we eradicated behaviors like starving and binging, issues of unhealthy beauty standards would nearly evaporate. That's the kind of social goal we should all be working to accomplish.