In no way meant to dismiss the pervasiveness of anti-fat sentiment, because it is definitely real

and, too

I wonder how often a novel describes the physique of its protagonist*, and when it does not, is "not fat" assumed.

*I don't read a lot of novels, preferring non-fiction and only having so much time in life

Also, Decoder Ring podcast had a similar look at fatness in visual media in its "Why Are We Still Using Fat Suits?" episode.

Some of the reason we use fatsuits is that acting is a surprisingly strenuous and difficult job. Finding someone who is truly obese enough to accurately portray obese people doesn't usually have the actual physical ability to act for a living.

George Cooper Sr (Sheldon's dad on Young Sheldon) does a fantastic job of showing a regular old overweight guy. He probably qualifies as obese under the medical terminology, but he's hardly what we think of when we think "fat person".

Can't say I'm surprised.

There's absolutely no way to show "fatness" as a positive except in very unique or contrived scenarios. Sure, being a little overweight is protective if you need to recover from a physical trauma, but what the average american pictures when they think "fat" is a body type that is indicative of either other health issues, impulse control issues, an unhealthy relationship with food, and just general lack of well being.

Given these negative generalizations about the state of fatness, you don't see a lot of it in novels.

Another contributing factor in my mind is that the few subcultures where fatness is a positive are all either really toxic fat-acceptance movements (where simply posting a photo and celebrating some weight loss is considered to be willful harm to yourself and others), or sexual fetishism. Neither of these subcultures are doing much good to normalize fat being OK.