That's because he didn't/couldn't fight back. A lot of the times in domestic violence against men it's not that the woman overpowered him, but he just wouldn't fight back. It's incredibly humiliating I'd imagine too.
You can't come out and tell people your wife beats the crap out of you. You're just forced to take it. People will laugh at you or won't even believe you. I remember I knew this guy who was tall and skinny, 6'1 and like 130-140 lbs who got beat up by his much shorter girlfriend who weighed like 30 lbs more than him. He could have easily beat her up but he didn't want to fight back.
What's even worse for poor Johnny is that it's his wife, if he so much as hits her he will lose out. Civil court cases are always advantageous to the wife, rarely can the man win.