There’s a reason why the tagline for my blog mentions size acceptance. While my primary focus is on fat acceptance and health at every size, I would generally recommend the same things for skinny people that I would for fat people. I believe that, even if they are “underweight”, in most cases the best thing that skinny people can do for their health is listen to their bodies: eat when they’re hungry, and do exercise that feels good (including in a “challenges are fun!” sort of way) rather than pushing through pain. And body-shaming of any kind is just plain mean.
But I agree with posts like this that skinny-shaming and fat-shaming are not the same thing. They are both bad, but they are not equally bad because they are not equally powerful. One has the weight of fashion, Hollywood, and the moral panic about obesity behind it, and the other does not.
If they were equally powerful, when people wanted to hurt someone thin (but not runway-model-thin) like Cassey Ho, they would mostly throw “you look anorexic” and “eat a sandwich” at her rather than telling her how fat she is. Tellingly, after she photoshopped a selfie to look thin, she got a “still too fat” comment. I don’t think I believe the person writing it really thought she still looked fat in the picture; I think they said it just to hurt her. But that is more proof of how much power fat-shaming has. That is why it is one of the first weapons people reach for.