I've bought a kitten, who seems a little smaller than his brothers - does this matter?

Basically the answer is "No, it doesn't matter"

I have a new kitten, she is 5 months old now. She may not have been the runt, there were 4 kittens when I first saw them and they all looked about the same size. Although I was told that they (the other kittens) push her out of the way when it's feeding time.

The important thing is to make sure that the kitten is fed kitten food. Adult cat food lacks the necessary minerals, vitamins necessary for growth.

I feed all my cats (5 ranging from about 13 yrs, 1 at 3 yrs, 2 at 18 months and the new one) Hills Science Plan for adults. This is a dry cat food pellets. At the weekend, all the cats get wet food (tins or packets) the young one (Snowball what else do you call a cat that is black all over) gets kitten food. She has a healthy appetite and generally pushes the others out of the way if she wants more. She is growing up really well, and shows no signs of being pushed around for the first 8/10 weeks of her life.