Calories in / calories out is not an interesting question. If you present a healthy person with 4000 calories of cake, they don't eat it. They may have some cake, but not all of it. Oftentimes, they simply can't eat it all without feeling sick and stopping. Healthy people constantly pass on adding extra calories to their diet, and it's not through willpower. Healthy people have different hormonal balances, different gut bacteria, different brain structures, etc. They simply cannot eat the same quantity of unhealthy food that an obese person can, and they do not have the constant level of background hunger cravings that obese people do. Changing a healthy person into an unhealthy one takes a while, but several months of being sedentary and eating low quality food generally seems to put someone on track for obesity and metabolic syndrome.
Conversely, an obese person, or particularly a formerly obese person that has recently dieted, has pretty extreme cravings and withdrawal symptoms, even if they're eating a complete and healthy diet. Something is going wrong with the complicated dance of insulin, ghrelin, leptin, and various others. Something is driving obese people to eat enormous amounts of food and still feel hungry. Poor sleep, chemical exposure, change in nutrient content of easily available foods, are all potential culprits. Something far more interesting is going on than "they're eating more calories".
