It always surprises me when people freak out about their kids not eating enough. My little one has gone through eating like a champ at six months then evening out to normal amounts, then eating like a bird for a good two or three years, and now he's putting down two bowls of oatmeal and an english muffin every morning which is what made me think about this. They eat when their body needs it. Usually when they're going through a growth spurt. Other times they eat hardly anything and sleep a lot, another version of growing. Sometimes they just don't eat very much at all. I've always been okay with that. If two bites at dinner works for them, that's fine. If they're hungry enough for dessert then they need to eat more otherwise they can eat two bites. The important part is making sure whatever IS going in is healthy - so if he's eating like a bird, make sure he's eating like a healthy bird.
I heard somewhere that your stomach is about the size of your fist - an adult stomach about twice the size. If you think about their stomach being that small there's no way a toddler can eat their whole sandwich or their whole hamburger.