I always wondered this as well! My first boy is ENORMOUS. He's a good 4-4.5 inches head to tail, and his tail is NOT that huge or long (HM, so it's wide, but not long). His body is as big around as my index finger and he's NOT fat, he's just huge. No one else has ever gotten as big as he is (though all my guys are large, like 2.5-3" long from head to tail, but a lot of that is tail), and I always wondered why he'd gotten so big. When I first joined the forum I asked this same question, and I did have a couple of people suggest that he could be half king.
I do feed him more than the others, quite a bit more in fact. He probably eats seven or eight pellets twice a day, but I always check him before feeding to make sure that he's not tubby. If he is/was, then I'd cut that number down. Plus, my guys live in 10g's, so they do quite a lot of swimming and have good muscle tone, so they're using what they eat. If he was living in a smaller space or wasn't moving as much, I'd probably need to feed him less.