Apart from genetics - he is the male firstborn (likely) - he also eats a lot of canned food and cooked food that is basically protein, also he eats many times a day, and he plays and sleeps a lot as well; all these add up to a bulky cat.
We really want him to be a big boy! Male tom cats are so handsome because of their size and muscles. Andy and I often admire the stray cats that are huge and handsome, and now we have one in the making right in our own home. If he weren't picky with his food, he would be even bigger.