Understand that there are necessary and sufficient conditions. A sufficient condition for obesity is too high chronic caloric intake. A necessary condition is retention of this surplus. Some people will feel correctly full when they overeat high calorie foods, others only if volume is too big, yet others have the distended stomach where even that does not work. Some have hormonal deficit that makes them retain more (e.g. hypothyroidism) or counters satiety signaling. Others have very wasteful metabolism or digestion where less is retained. Some really do highly strenuous activities that just demand more energy.

Ultimately you set a caloric target with a carefully constructed diet and watch the long term weight slope over months. That's the science of it. Even high fat ad lib diets ultimately resulted in caloric input restriction, stronger than other diets they tried. (see Cambridge study)