Prior generations didn't try to maintain this kind of build, it came from a variety of factors:

1.) Healthy natural foods grown locally, not over-processed shit imported from China/Mexico full of corn syrup, microplastics, seed oils, etc..

2.) Everyone walked everywhere, you lived in a city and went to places nearby and rarely ventured outside of that. Cities were still safe and infrastructure/public transportation was state of the art.

3.) Your life was non-sedentary. You were on your feet all day at work, you went to parties and schmoozed around, you went to clubs and danced, you went on afternoon and evening walks, you went to the public pool and swam, you played tennis or baseball or golf just to kill some time, you rode bikes, you went to bars and cruised for women. You used your bedroom really just to sleep.

4.) US dietary habits were European still as opposed to Latin American. People ate lean red meat, chicken, fish, vegetables, dairy, potatoes etc. instead of just bread, carbs, rice, and fried like now. When combined with the aforementioned lifestyle, a classically European diet of meat, dairy, and vegetables produces a naturally lean and robust build as opposed to the quick-burn high-fat storage intake seen with Latin American/African American/Asian foods.

A myth of the obesity in the US is its about from too much food/portion. Maybe that was initially true, but today your average American has less quantities of food available to them than Americans 50 years ago due to the enormous cost of food today in the US and the relative scarcity of it (notice your supermarket shelves are always way more empty these days vs. even 5 years ago?). Portions in the 1950s America were probably the same if not larger than 2020s America, and you could get it for far cheaper even adjusted for inflation.

5.) Your appearance mattered more. If you were fat and didn't dress properly you were socially shamed and ostracized. This trait still exists in places like Japan/China.