The only thing I can think of is vestigial cultural rules being a determining factor; what you eat, when, how much - what your hobbies tend to be, how much you move in life.

Every individual of course sets his or her own path, but populations taken as a whole seem to exhibit dominant patterns.

So the only thing you can think of to explain this is cultural factor?

You don't think genetics could be a factor here at all?

Sorry. I claim zero knowledge of epidemiological obesity studies or theories. In retrospect, it was not my place to conjecture at all --- I really don't know anything about this.

Although for a thoughtful and thorough analysis:

An introductory wikipedia article: https://en.wikipedia.org/wiki/Epidemiology_of_obesity

Google scholar search: https://scholar.google.com/scholar?q=epidemiological+obesity