I do wonder if its because one is bigger in the cats womb - and the other cannot grow. I have had it with a litter of pups too. One of the male pups was absolutely enormous, and the other was completely the opposite, so small. The two female pups were normal sized. When they grew up, the bigger pup was always taller than his brother, but not abnormally so!! Will try and find the pic of our big and small kitten later.