as far as the study goes, that's based on reported cases. Simple facts are that most women can hit me as hard as they want, multiple times, and I won't be hurt badly enough to bother reporting it. Remember, not all women are built like Chyna, and most men are significantly stronger than most women.
Example
Woman hits me as hard as she can, I flinch maybe get bruised, possibly bleeding slightly.
I hit woman as hard as I can, she has 3 teeth knocked out and a broken jaw.
It's not that physical violence happens less from women, it's that it doesn't hurt me, so why do I care.