It would also be informative what kinds of people find what kind of advice sanctimonious and condescending.

You also have the effect of non-overweight people who enjoy being sanctimonious or condescending.

I agree that's a huge problem. On most discussion boards, when the topic of obesity comes up, there will always be a couple guys who show up to say something like "Just stop stuffing your face and you'll lose weight." Which typically generates a lot of bickering. Still, one can ask what is going on psychologically in such situations.