I don't want to speculate about EY other than saying my model of him didn't expect to ever see "too complex" brought up as a reason not to try anything. Maybe there's too much overlap with HPMOR:Harry. It is possible that there are individuals whose akrasia levels / mental energy and self-image are unaffected by being overweight compared to not being overweight. Just unlikely.

As far as I've heard, most of the health benefits of exercise can be gotten without needing to actually be thin.

That's true, but doesn't change that ceteris paribus given little exercise, you'll still live longer not being obese. Note that I've not even mentioned exercise. It's certainly better being overweight and exercising (while still being overweight), than being overweight and not exercising.