Most people have unused capacity in their penis, or a weak erectile pressure system. So, in-order to make things more efficient you will have to expand your pipes, or the blood carrying tissues. When this is done, you will experience larger erections; that unused capacity will be realized to a degree and the improved efficiency of the erectile system will spring larger boners. The penis vessel or cylinder, will hold more blood and as a result the volume of the penis increases. This is usually a 3 to 5 fold increase, so its quite dramatic.