Why is my body like this?

He is a man. Men naturally have higher matabolosms than women do, because 90% of their weight comes from muscle, ergo, most males are, yep you guessed it, only about 10% body fat. Muscle weighs more that fat and takes up FIVE TIME LESS SPACE! So, your brother is very thin because he is all muscle and since muscle is a furnace, lb for lb, he is a walking incinerator. WHatever he eats burns up on the way down.