My 2c is that one should simply look at the diet of lifestyle of the fat people they know, and the obesity problem will become clear.
Tons of soda. Products like Hamburger Helper instead of actually making pasta and sausage yourself. Extremely high volume of food in general. Constant, constant snacking. No exercise. So a massive amount of low quality food and nothing to burn it. Cancer, heart disease, joint pain, diabetes, heartburn, everything else.
Occasionally you run into some outré example like a family I know who eats exclusively home-cooked meals with grass-fed beef and venison yet are all overweight, but I've had lunch with them and that was some of the richest, tastiest food in my life plus all the trimmings. Also, they don't really care about exercising, or for that matter their weight.
I don't think this is complicated. Weight gain is simply calories in and calories out. The only contentious part is the psychological compulsions that cause people to eat amounts of food that they know are bad for them, sometimes while maintaining a sedentary lifestyle, which they also know is bad for them.
I'm sure it's not as simple as that unless you only mean people starving themselves.
It's not. Hormones (insulin especially) play a huge role, and the body can actually be reluctant to shed unhealthy fat if it thinks it's going through a famine. That's why (whole food) keto and carnivore diets are so effective at weight loss despite to renouncing portion control completely.
People effectively lost weight long before keto, and you don't throw portion control to the wind on a keto diet. Hormonal imbalances can cause weight gain and those should be checked, but maintaining a caloric deficit will always result in losing weight. It's impossible for it not to do that.
It's far more what you eat, not how much.
OP already addressed that when he mentioned calorie-dense foods.
There's a History Channel competition show called Alone where they drop people off in the woods with some basic supplies and whoever lasts the longest wins a big cash prize. If it was more than calories in/calories out, every single competitor wouldn't end up losing weight in the exact same way at the exact same pace.
It is as simple as that. The body can't store energy that you burn or simply don't eat. Otherwise you would violate thermodynamics.
Give me any fat person, let me run their diet and exercise, and they will gain muscle and lose fat.