I'm uncomfortable with the notion that most people are subconsciously choosing their weight, because that implies if you are big, there is something wrong with your head.
There are enough people who think there is something wrong with my body because I'm big. I don't really want them to all think there is something wrong with my head too!
Relating it to sexual orientation: At one time I had all sorts of psychological theories about why I couldn't make up my mind whether to be with men or women. Then a smart therapist said "Maybe you're just bi." I realized all my theories were unnecessary -- I didn't have to make a choice.
In other words, having plausible psychological theories about why one is big doesn't necessarily mean that's why one is big. It means that one feels the need to justify being big. That's understandable in a society that believes there is something wrong with being big.