I've come down on the position that Americans are fat due to malnutrition. My key phrase is filter feeding.
As far as the body is concerned, calories are just another kind of nutrient. If you can't get enough manganese but have plenty of calories, you keep eating until you get enough manganese. If you have plenty of manganese but not enough calories, you keep eating until you get enough calories. The issue is that farmers are being subsidized (everywhere) which means their plant husbandry is dysgenic. Farmers have bred plants for appearances, strongly to the exclusion of substance, in the apotheosis of the American style. Estimates say plants have lost 90% of their contents, though it's probably more like 40% or 50%. It's a lot, but American are hyperbolic in addition to being obsessed with appearances, and the hyperbole ought to be deprecated.
(E.g. brussel sprouts are still weakly bred, meaning they taste funny but are still sort of nutritious.)
Since nothing has nutrition, it's all junk food. There is quite possibly nothing you can get in a grocery store that isn't junk food.
Whales have to drink enormous amounts of water to find a small amount of krill. You have to eat an enormous amount of plant material, or meat fed on farm cultivars, to get a small amount of vitamins and minerals, and as a result become whale-esque.
The key datum is that lab rats are also getting fat. It's definitely an environment thing, not laziness or other genetic problems. Maybe it's a ferociously poisonous and ubiquitous airborne pollutant. Or maybe even rat chow is made of plants, not from chemical feedstock, meaning they, too, are suffering malnutrition these days.