Fat Fu and Red No. 3 point out that the reason someone gives often seems to have more to do with that person’s ideological leanings than anything else.
To be fair, Fat & Size Acceptance people have their own ideology. I suspect that dieting, our screwed-up relationship with food, and our fatphobia contribute to the obesity epidemic (such as it is).
Still, I’m reminded of how Fat Acceptance are said to be denialists, and are sometimes compared to global warming and/or evolution deniers. One of the arguments against those two types of denialism is that if they had something going for them, there would be evidence in favor of one alternate theory, rather than a bunch of alternative theories which are in many cases mutually exclusive. They would have a particular competing explanation, rather than throw out a bunch of ideas and see which ones stick–because that tends to be the mark of someone looking to believe something (or not believe something), rather than someone who actually has a good alternative explanation.