Slow (stopped) growing chicks?

I have 40 or so chicks at 4 weeks old all fine and healthy however one is half the size, maybe even smaller. It seems to have stopped growing but seems happy enough and sticks up for itself too. I have a couple of theories but don't want to suggest anything yet as I'd like people to consider this with an open mind. Thanks in advance people!

Maybe its not getting as much food as the others - bigger chicks shoving him out of the way when he's trying to feed or maybe it genetic and he carries whatever gene makes the mini japs and finally maybe he's just a runt ... OK end of my guesses

I hope he makes it though often chicks that fail to thrive die in the end ... so fingers crossed.

I just put it down to natural variation in size if the others are growing normally

Thanks, I had thought it might be due to inbreeding but this is unlikely as I buy in a good proportion of birds from other sources. The little guy is fine and seem to punch above it's weight which is encouraging.