The diameter of the head is greater front to back than side to side. Most helmets are more or leass round, and will therefore have a greater overhang at the sides than at the front and rear. In addition, helmets are not designed to sit close to the head, but to have a liner between helmet dome and head, to help absorb any blows. If you look at re-enactors wearing helmets you will see a variation in helmet fit and overhang.
It looks fine without the left arm, but would look better with the left arm holding a shield, mounted on a pedestal.
Best wishes,
Mike