I don't think a large proportion of thin people are thin because of societal pressure. And conversely I don't think societal pressure makes many people thin, because I hear so much complaints about body shaming and unrealistic beauty expectations and discrimination against obesity etc, so it clearly isn't working any miracles.

I think most people who are thin just have a food intake regulation that is pretty well balanced so they don't over eat because they don't feel that hungry when they have had enough calories.

The reason why some groups of people have been increasingly prone to obesity is external factors interfering with that regulation. It's probably lots of things, food availability, ingredients, cost, culture, other mental health issues, medications, entertainment, work, availability of cars. One thing it is not is simple.

The calories in vs calories out mechanic is simple, the reasons why that's going out of kilter is not.