In my experience, it seems to be the people that “eat whatever they want” tend to have one huge meal in their day, so it only SEEMS that way!
So basically, they’re grazing all day except that huge meal of fast food where they get all their calories, but it’s still not enough for significant weight gain!
Their portion sizes are also MUCH smaller outside this meal.
But that’s just what I’ve seen in my field
Helping people “eat what they want” while still improving their health is what I do professionally.
There are a lot of layers to answer your question, but it’s most easily answered with more questions.
Why do some people have brown hair and no matter how many times I dye mine it still grows blonde?
Why do some people have green eyes and everytime I take my colored contacts out mine stay blue?
It feels unfair because of the moral attachment to size in our size bias and stigmatized culture.
Our body is in charge of our size (not us) and regulates it based on infinite inputs it receives and interprets not just on a daily basis but over a lifetime.
The only thing we have control over is how well we take care of it.
Weight isn’t as simple as energy in & energy out. There’s a lot more to it and more importantly to health than we’ve been conditioned to believe.
Because of that, many people may not like my comment. But, I’m saying it anyway because my life experiences have shown it to be true.