One of the common tropes in science-fiction is strong women - Molly Millions, Buffy, Eowyn, Lara Croft, Leeloo - these are all women that know how to fight, know how to plan, and never shirk back from anything. I can't get enough of them. But here's the problem - these women are typically more beautiful than buff. They have lean figures, little muscle mass, and small statures. Are these woman seriously going to take out someone like The Big Show crossbred with Andre the Giant infused with demons, cybernetics, and rings of power? This was the question proposed on Jim C. Hines journal (and associated links found therein) a few days ago, and I thought I'd add my two cents.
I won't discuss much about the issues of plausibility. That's already been discussed, and I believe that in a fair fight (i.e., no guns or chainsaws), anyone can defeat anyone, as long as they have the skill and know-how. There's enough nerve-clusters and weak points in the average human body that can bring a man to his knees, even if you don't kick him in the pills (that's just not cricket). And even if its not a fair fight (which it usually isn't - where's the fun in that?) there's usually some plot device that saves the hero or he gets out by his wits or luck. We've all seen it before - Luke Skywalker is ridiculously overpowered by Vader, then Emperor Palpatine, and is saved by a change of heart (not his - hee hee). The Karate Kid manages to defeat the school bully with some ancient Chinese secret.
But I didn't come here to argue whether a fighting woman has to be buff to hold her own. My point is that arguing is moot. Have you ever seen women weight-lifters? They are scary. They're like genetically engineered monsters - all oil and vains. Their boobs look like teardrops on their chest. We'll never see these women kicking demon ass, because they're as scary as the demons they're fighting. We (meaning women too) want women to look appealing, especially if (as they usually are) the love interest for the main male. We like all different types of woman, but we don't want hulking brute masses. I couldn't tell you why - I'm sure there's a psychological reason for it. For me, I don't like women that don't exhibit some kind of vulnerability or motherly instinct. There's something in my psyche, perhaps the male psyche, that drives to protect. Each of the women I described above weren't invulnerable, and needed someone to help out every once in a while, usually a man, because it keeps the dichotomy going. Being small and soft helps this trope.
If a woman is a lumpy mass of flesh, they don't need protecting. They don't exhibit motherly instinct. And they don't even seem capable of love. I'm sure that's not the case - I'm just talking about impressions. And impressions are often all we get from stories, because anything else slows the story down.