shouldn't a guy hitting a girl be just as bad as a girl hitting a guy, not counting the power of the hit?
Yes. Ultimately, they should rank the same. However, that doesn't mean that they are the same today. Violence by men against women is often an attempt by these men to assert their dominance, and frequently entails considerable physical injury and mental trauma on the part of the woman.
To put it another way, it isn't really about who is hitting whom, but
why. Sometimes physical assault is acceptable, for any number of reasons. Much more often, it is unacceptable, again for any number of reasons. And, sometimes, physical assault is
absolutely unacceptable, without qualification. All sexist violence falls into this last category, especially when there is an intent to dominate, injure, or sexually assault.