That's not really limited to fat people though.  Anti-vaxxers, climate-change deniers, 6-day creationists...people constantly think that "common sense" is more valuable than actual, proven knowledge.    I really wish we'd stop focusing so much in schools on the science that was wrong back in X time period because it makes people think that they are right to question it at any given point with zero training.  No, just because we didn't know what a germ was a few hundred years ago doesn't mean you being fat proves conservation of energy is wrong.