Men are (for obvious reasons) less likely to seek medical attention or call the cops unless the injury is incapacitating. Plus, being generally larger and stronger (I said generally - don't flame me), an equivalent amount of force will cause greater damage to a woman than a man.
Doesn't mean the women aren't trying!
I agree with this completely. For a variety of reasons, domestic violence is underreported on men, as the other poster workingclassfilth aptly stated.
I would guess many, if not most men are not going to press any kind of charges if they were assaulted by their wives, but the reasons may often be different than those given by a woman who refuses to press charges.