There is no "average size" of the male cranium. A lot of factors determine head size, including ethnicity. We are all very different. To determine an "average" size would leave a lot of people feeling freakishly small or large.
If you've shopped for men's hats, you've noticed it's probably easier or harder, perhaps to find the medium-sized fit. This only a generality but men's head sizes vary. It's genetic.
