Survival of the fittest is bogus in so many ways. Next time someone evol-psych fat shames you, or uses it to justify any other prejudice, I recommend calling up this thought experiment of David Deutsch’s

“ Imagine an island on which the total number of birds of a particular species would be maximized is they nasted at, say, the beginning of April. The explanation for why a particular date is optimal will refer to various trade-offs involving factors such as temperature, the prevalence of predators the availability of food and nesting materials, and so on, suppose that initially the whole population has genes that cause them to nest at that optimum time. That would mean that those genes were well adapted to maximizing the number of birds in the population - which one might call ‘maximizing the good of the species’.

“Now suppose that this equilibrium is disturbed by the advent of a mutant gene in a single bird which causes it to nest slightly earlier - say, at the end of March. Assume that when a bird has built a nest, the species’ other behavioural genes are such that it automatically gets whatever cooperation it needs from a mate. That pair of birds would then be guaranteed the best nesting site on the island - an advantage which, in terms of the survival of their offspring, might well outweigh all the slight disadvantages of nesting earlier. In that case, in the following generation there will be more March-nesting birds, and, again, all of them will find excellent nesting sites. That means that a smaller proportion than usual of the April-nesting variety will find good sites: the best sites will have been taken by the time they start looking. In subsequent generations, the balance of the population will keep shifting towards the March-nesting variants. If the relative advantage of having the best nesting sites is large enough, the April-nesting variant could even become extinct.

…

“This change has harmed the species, in the sense of reducing its total population (because birds are no longer nesting at the optimum time). It may thereby also have harmed it by increasing the risk of extinction, making it less likely to spread to other habitats, and so on. so an optimally adapted species may in this way evolve into one that is less 'well off’ by any measure.

…

“Evolution can even favour genes that are not just suboptimal, but wholly harmful to the species and all its individuals. A famous example is the peacock’s large, colourful tail, which is believed to diminish the bird’s viability by making it harder to evade predators, and have no useful function at all. Genes for prominent tails dominate simply because peahens tend to choose prominent-tailed males as mates.

…

“If the best-spreading genes impose sufficiently large disadvantages on the species, the species becomes extinct. Nothing in biological evolution prevents that. It has presumably happened many times in the history of life on Earth”

The Beginning of Infinity, “Creation”, 89-92

I’d argue fat genes are some of the best-spreading among humans. And it clearly isn’t enough of a disadvantage to make our species extinct.