Mine always vary in size, it seems to depend on the size of the eggs they hatch from. I haven't noticed any relation to chick size and gender and you'll probably notice that as they grow, everyone will sort of even out. I would be concerned about a chick that small being so aggressive tho. If it were mine, I'd mark it to keep track as it grows and rid myself of it if it turns out to be a rooster. If its very aggressive this young, I'd be concerned about that when it weighs 15 or so pounds!