It's really about fitness, and fitness is mostly about nutrition. Society however at some point decided that calorie deprivation and dehydration is the way. If you are a woman you have to look like a twig (thanks fashion industry) and if you are a man you have to look chiseled (thanks romance industry). It doesn't matter if you are only eating vegetables, as long as you are thin you are considered healthier than someone who looks like a tree trunk and can eat an entire pig, but also lift a car.
Even if obesity has some amount of protective benefit for certain types of chronic death categories…it doesn’t follow that being “alive” is “living”. As my own anecdotal life has many examples of obese people that live a relatively long life but the last 30 years of it have been miserable…in that they couldn’t even enjoy simple things like a walk around the block.