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.
The question is why they have better balanced calorie intake. It's certainly not lack of sugar availability.
I loved the food, but it was not at all what anyone would consider healthy.
(Instant Ramen are also extremely popular, industrially produced fried noodles with way too much saturated fats + sodium)