Little birds and hungry hippos

I know that children will never starve themselves, but sometimes I wonder. How can they eat so little and still function? I know that children will stop when they are full, but still I'm confused. After second and third helpings and dessert, how can you still be hungry? There are days when I could get away with giving the girls table scraps at every meal and they would be satisfied. Then there are days when I feel like they are eating us out of house and home. As long as they are growing and healthy and happy, I guess I'll just stay confused.