I think you raise a very valid point and there are a probably ton of other factors that I’m not even aware of that go into this.
I’ll throw another othrogonal point into the mix: our definitions of “skinny” and “fat” are probably very different and very relative to our realities.
Anyway, I don’t claim to have the answers - it gets really messy to articulate the big question of “how do values form”.
Cheap food is why the norms played out like they did. In places with less easy access to calories, fat is considered beautiful and thin ugly.