Why do you eat so much but never get fat?

For some, it's in their genes. I have a friend who eats nothing but candy and junk food but she still retains a perfect figure. For others, they usually exercise so much that they burn most of the calories they've eaten. It also depends on kind of the food. Even if amounts of different foods are the same, it is its type that controls the weight. For example, vegetables vs. junk food make a great difference...