I think being fat would be kinda hard for most people, even if they aspired to it.
For one, most (sustainable) food is unprocessed (albeit mutated) - no added sugars/corn-syrup. It's also more fibrous, making it more filling for less calories.
Second, food is not available in excess for most of the Commonwealth. You have to earn the right to eat in most places, especially since a lot of the food is too infected or irradiated to safely consume (regardless of how mysteriously plentiful Fancy Lads is after 210 years).
And three, physical activity would be much higher post-apoc. Whether you're walking long distance as a courier or scavver, doing manual labour as a farmer or mechanic, patrolling as a guard, or actively hunting the wastes - you don't have the luxury of sitting in one place waiting for pizza to be delivered.
The only place I think you'd see more hefty people would be in functional vaults, since they one, are stocked with Pre-War resources; two, have mechanically-/computer-controlled growlabs designed to support their population; and three, will include professions that don't expect much physical movement (most notably, desk jobs). And I do remember seeing a man in 81 that met that expectation.