From what I've read, the consensus seems to be that the force of a stretch, for example, will be at a higher "concentration" with a smaller penis than a larger penis, because the force will be distributed over a smaller area. When I was first starting (about 5 bpel) I gained very quickly. Over time, especially after the 8'' mark, the gains slowed down considerably. However, what I experienced was most likely a result of a conditioned penis. In my opinion, smaller penis = quicker gains. A lot of the time, thinking and believing that to be true will be all you need to speed up gains =D So, smaller penis = quicker gains!
Happy gaining,
-Doc