However, that said - on a wider scale, let's not pretend society treats fat and very thin women equally badly - for one thing, consider that I used "fat" and "very thin," since you don't have to be "very fat" to be broadly condemned for being fat, period.

This, definitely. Our culture does not give the same crap equally, even if they are both inappropriate body policing. One of these forms of body policing is not just saying, "how you look is my concern and you should be very interested in my opinion" (which both fat and skinny women get, because women get it) it's also saying, "you are bad and you should feel bad" (which is pretty much reserved for fat people).

It's like how calling straight people "breeders" is not as bad as calling gay men "faggots." They're both rude, and share a reason for why they are unacceptable, but they are not even near the same league of badness. It's just damned difficult to effectively insult someone for too closely adhering to what is culturally approved, because if they reply in kind they'll have a lot more power behind the swing. Not equal at all.