Why would a certain number of them, like the ones in the middle of the biceps, suddenly start to grow differently or at a faster rate than its partners? If anything, the muscles that are closest to the insertion points are the most prone to mechanical stress, and you don't see them get any bigger than the rest of the muscle. If they did, everyone would have proportions like Popeye. This is true of any muscle, but you're probably thinking, what about quads? I know that when I do hack squats with my feet together, it tends to give me more sweep in my legs.