Can kittens from the same litter vary in size?

Of Course! it is perfectly normal, just like it is for humans. if your dad is tall and your mom is short, you little sib could be like, a foot shorter than you. its the same way with cats. the parents may have been different sizes, so one of the cats just picked up a gene that the other didn't. i would still take him to the vet, though, you never know when something could be wrong. but, i wouldn't get too worried about it. Hope I Helped!