This is the key. On evolutionary time scales, human beings have only very recently come to inhabit a world where regularly being able to eat to full satiation (and beyond) is commonplace. Being "a little hungry" is actually the typical condition under which most of our ancestors over many thousands of years have operated. Put an organism evolved to survive amid scarcity in an environment of abundance, and it's going to gain weight unless it comes up with a method for stopping at or before the earliest signs of satiety.
This is the key. On evolutionary time scales, human beings have only very recently come to inhabit a world where regularly being able to eat to full satiation (and beyond) is commonplace. Being "a little hungry" is actually the typical condition under which most of our ancestors over many thousands of years have operated. Put an organism evolved to survive amid scarcity in an environment of abundance, and it's going to gain weight unless it comes up with a method for stopping at or before the earliest signs of satiety.