Yes, it’s definitively stupid to speculate that obesity is due to poor diet while the elephants in the room are over-eating, lack of physical activities, smoking, binge drinking... But it’s so political correct to put the fault on others (the industrial complex, the processed food whatever-it-means, the conspiracy du jour...) instead of implying any personal responsibility.
You want an obese to get good health ? Tell him to eat exactly the same “junk food” but only half in quantity and exercise.
I always say, stop eating when you start to feel full.
I wasn't sure where you were headed, but you are dead on. When my wife was a teen her doctor told her she was getting to heavy. She asked what she should do and the doctor told her to eat half as much and not try to finish everything on her plate if she felt satisfied. He was a highly respected physician in our community who was one of the founders of our children's hospital.
My wife has been slender ever since. It was a little unnerving to me when we first met because I was taught from an early age to always eat everything on my plate. She often doesn't finish everything. Since we have been married for 30 years I often eat what she doesn't finish. And if we go to a restaurant we always take home a doggy bag.
Which brings up another important point. Restaurants nearly always serve larger portions than what their customers actually need. We have some very large relatives and they all love to eat out. I don't think it is a coincidence.