People who are compulsive masturbators usually have small heads is because when they masturbate it stresses the brain dramatically which the stress does affect the growth of the brain and when you put too much stress into the brain it will shrink your brain dramatically causing your head to shrink in size.
Also it has a scientific affect on your muscles that makes it smalller not just your penis but your entire muscles in your body. So my advise to all of you is dont masturbate its not healthy if you do it 3 or 4 times a week. It also makes you stay home alot and it destroys your social life.