When i was much younger, i ate until i was stuffed. Life was so fucking good. The last few bites aren't as good as the first, but the amount of goodness from the point where your hunger was filled to where you're stuffed adds up. It's even better when you live with your parents and the house is fully stocked with goodness.
When i started to care about what i look like, i ate until i 'wasnt hungry'. Eventually when you do that, it takes progressively less and less to make you full. It's weird, because whereas at the age of 16 i used to be able to eat a medium pizza without being totally full, i can have two -three slices and it'll be all over (at the age of 21), though i rarely eat pizza now anyways.