How are all her opponents undersized? She's fighting at the same weight as them. If we think Rousey is too big for the weight, we can look at plenty of guys in almost every UFC division and claim they're too big for the weight, despite making that weight.
I disagree, I'm not with the notion that "If you make weight it's fair game", never have been. There are clearly fighters in a division that hold a significant weight and strength advantage over others. It's legal but I can't give too much credit for that. Especially in women's division. Cyborg is like that, she makes 145, she enters the ring at probably like 155 or 160 and you can see the size advantage. Same thing with Ronda, she's clearly way bigger than the other 135 lbers.
That's not really the point. The point is the women just do not display the level of skill of the men, that's just the truth. The main problem like steelcityroller said is it takes up space on cards.