If you believe that "Calories In" and "Calories Out" control how much fat we accumulate in our fat tissue, then obesity can only result when someone eats too many calories ('overeats') and/or burns off too few calories ('under exercises'). Eating and exercising are both behaviors. Ergo, obesity is a psychological problem. It's driven by some defect in the brain.
And indeed weight loss solutions are almost all premised on this idea. Change behavior, and you'll change how much fat you accumulate. Cut back on the amount of food you put in your body (i.e. lower "Calories In" by eating low calorie diets) and increase the amount of exercise you do (i.e. raise "Calories Out"), and this will CAUSE you to mobilize more fat from the fat tissue than gets deposited there.
Let there be no doubt: the Caloric Balance Hypothesis leaves no room -- no room whatsoever -- for physiology to be a factor. You get fat because you're sedentary or you overeat. And you lose weight because you exercise and you eat low calorie diets. There is nothing more to it.
Consider this quote from an American Journal of Psychiatry article "issues for DSM-V: should obesity be included as a brain disorder?"
"standard interventions based on promoting lifestyle changes to decrease excessive food consumption (dieting) and increase physical activity (exercise) are effective and can normalize weight if followed rigorously, but unfortunately they are incredibly difficult to sustain..."
This article suggests that dieting is fundamentally an exercise in appetite control and willpower -- activities that researchers presume to be regulated by the brain.
Here is another quote from researcher Rena Wing of the National Weight Control Registry:
"There is no way around it... if you want to lose weight and keep it off, you really need to change your lifestyle, particularly if you're overweight or have a family history of obesity. Our data... suggest strategies associated with successful weight maintenance include high levels of physical activity and conscious control of eating habits."
Once again, bad behavior is presumed to lead to obesity, and good behavior is presumed to lead to weight loss. This is an incredibly important prediction, and it is essential for the Caloric Balance Hypothesis. If you could provide evidence or show data that psychology is not important -- or even that physiology is even partially important! -- it would be a huge problem for the Caloric Balance Hypothesis.
To reiterate: the Caloric Balance Hypothesis tells us clearly that, when we gain weight and can't shake it even on low calorie diets, it's our fault; or, rather, it's the fault of our brains. Our bodies, our fat tissue, our metabolism, our hormones -- they have nothing to say in the matter.