Why is Bestimus so small?

Bestimus is one of three kittens that I took in when they were 4 weeks old.  He's now 4 months old.  He's been wormed, vaccinated, he's negative for FeLV and FIV, and is otherwise healthy, active, and bright.  All three kittens had an upper respiratory infection, but it's cleared up now.  

But, compared to his siblings, he's tiny!  I just saw his brother and sister last night (they live with my sister), they're twice his size!  When they left my home, they were all the same size.  What's going on?  He gets however much food he wants.  He's a little on the skinny side in addition to being small, and the vet said I should give him a few more worming treatments just to be sure, but otherwise he can't see anything wrong.  

Bestimus gets the same food as my other cats (low-carb wet food - no single brand we stick with) except I sometimes put KMR and salmon oil in Besti's for extra calories.  My sister's feeding the other two Friskies wet.  

I don't get it!  Help me out here.  

I have 3 cats that were hand raised fro the same litter 2 are 12 lbs and normal sized one is 4lbs and very tiny.  When I have taken her in to vet people think she is a kitten.  Nothing wrong with her and she is 9yo.  As long as he is gaining some and not having any issues keeping food down he may just be on smaller side when he is full grown.  I had a kitten with a liver shunt who could not gain the minute she started anything but milk.  But it was obvious there was an issue as she acted sick.

I also have a litter I bottle raised, 3 females, and 2 of them are big hefty girls (9-10 pounds) and one is a teeny slight little thing (5-6 pounds). Very active and healthy, just a small cat. So he could just be a smaller cat, or maybe he's just a late bloomer .

I haven't been weighing him myself, although he had gained weight between vet visits.  I guess we'll just see if he picks up the growing.