I believe it goes beyond merely stomach size. How much someone eats is also influenced by their level of appetite / hunger before triggering the response to eat. I go through phases where I make a decision to eat less (and feel a bit hungry) and after some days it becomes normal to the point it no longer triggers me to eat. My appetite is recalibrate. So I feel there is a substantial mental aspect to it beyond the shrinking and enlarging of the gut.

And or course then there's the cultural habit / ritual of eating 3 times a day.