As someone from one of the most obese states in the union, the core of the issue is most definitely not portion sizes. Food quality is shit and most people will eat ultraprocessed foods on the daily because money is tight for most people and fresh/healthy food is hella expensive assuming there's even access to it. If you have the same amount of calories in ultrprocessed food vs real food, your body will still gain more weight from the ultra processed one because it is less satiating because your body burns through it much faster.

Small scale organic farming used to be the norm in my grandma's time, but nowadays younger people don't know what an okra plant looks like or cracked open a pecan. Additionally if you're even able to access and afford fresh produce it's mainly water and is not nutritionally dense. This is an issue with the entire food system that's been built on shipping stuff from far away and profit incentives. Not to mention the targeted advertising of ultra processed foods to kids...

Also, there's virtually no public transit and the small amount of pedestrian and bike infrastructure is laughable and outright dangerous. You have to have a car to get to most places and most places require a lot of driving to get to depending. That means sitting down for most of the day. It's hard for most people to get exercise in daily life without being super intentional about it, and a lot of people are overworked and underpaid on top of that, so they understandably just wanna relax. As with most things in life, the answer is always complicated.