If you have the plans for a house and you are in the middle of building it, and find out that the engineer wants a bigger porch, does that mean you put a porch on every wall? No, YOU JUST BUILD A BIGGER PORCH. Genes indicate how strongly they express by repeating a bunch of times. (though sometimes this acts as a limiter, hence why you need to find out if copies increasee or decrease expression) The number of the structures is a different gene, (though that works similarly) so If you wanted more dicks, you could do that, but it would be a completely different thing to ask of DNA vs size.

TL;DR: "Size" and "Number" are on different genetic sliders.