Good question, and probably a lot of factors. Perhaps one is that being fat vs fit is almost like a caricature of the differences in the sexes, where men and naturally leaner and more muscular vs women, so perhaps a woman being fat isn’t seen as that odd whereas a man being fat is, particularly as it diminishes the visible characteristics we associate with the male body, ie it ruins a v taper, makes a man physically soft, less physically capable all around etc. Perhaps something in our brains is hard-wired and/or socially conditioned to see that as undesirable in men; an obese man is unlikely to be the kind of man who could go on a hunt and bring food back for his family, or build a home, or fight alongside men of the tribe.

Along with that it reflects a lack of discipline and adventure I’d say, so it also likely informs people about a mans habits and choices in a way that reflects negatively on how his status is perceived, and that’s a big deal for guys both in terms of how they perceive themselves snd how others perceive them.

And honestly, and I don’t intend to come off sounding like an ultimate bass edge lord here, but I think it’s just that women almost coddle one another. I’m part of a social work group chat and I swear to god that every day the women in it have something akin to a pity party where they take turns venting and supporting each other over stuff. In theory being able to do that is great but they do it over minor stuff too and they do this every. single. day. I can’t put it into words very well but I’ve always noticed a difference in how men handle stress with one another vs how women do, and it essentially comes down to “let me help you fix this” vs “let me be here to make you feel better”. Both are good, and arguably sometimes one is better than the other, but I can see these mindsets reflected in how the majority of men and women have outlooks in life (ie women saying things like “don’t worry this will pass / things will fall into place” vs most men saying “don’t worry we can sort this out / things can be done to resolve this”). Writ large, I can see that mentality leading into a normalisation of “I don’t have to change” for women being more common than men, hence body positivity being more common amongst/almost exclusive for obese women than obese men.