It's so cute when people who know nothing of the law try to make legal judgments. The law doesn't require people to make instantaneous judgments about possible force based on weight, angles and vectors. The only question is whether a reasonable person would have thought that there was a good chance that the woman intended "batter" him (as in the legal term "batter"). A battery is defined as an unlawful "touching". And you can use reasonable proportional force against an apparently imminent battery, i.e. an unlawful touching. The law doesn't require that the possible victim make a judgment of "oh, this person probably can't hit me hard enough to put me in the hospital." As others have cogently pointed out, if this had been two guys, there is little question that the "guy" who swung at Johnson would have been charged with battery. As for the "he could have easily walked away" again, you are confusing a moral judgment with a legal one. The criminal law in most states used to require someone to walk away, if a reasonable opportunity presented itself, before using force in self defense. But even those laws usually only applied to use of lethal force in self defense. However, the trend has become to not require the "victim" to walk away before using any form of self-defense, even in instances of using lethal force. Thus, the so called "stand your ground" laws.
This. From a moral standpoint, no question he should have just laughed and walked away. However, the legal position is far less clear. I'm sure this will plead out, but were I the defense attorney, I would argue that even his last punch was a proportional response. You are allowed to escalate your defensive response if he initial response doesn't eliminate the danger. I would then argue that this wasn't a Ray Rice, knock her out, punch, which Johnson, as an good-sized athlete could have delivered had he wanted to. And I would want to try this case to a judge rather than a jury, to eliminate the "oh, she's just a girl" moral judgments. Yeah, that defense probably would not fly, even to a judge, so that's why I anticipate this pleading out.