It’s certainly possible for a 115-pound woman to bring down a grown man with a punch or a kick if they know what they’re doing. Hell, my daughter weighs 48 pounds and if she kicks you in the nards the right way (or the wrong way, from my perspective) you won’t be up for further combat.
The problem is that the way the scenes are filmed, it doesn’t even look like the mooks are getting hit hard. If it was choreographed in such a way that Summar Glau actually put her weight and strength into a punch or kick into a man’s nuts or his solar plexus or the bridge of his nose, I could believe he was hurt. But in scenes it’s often a flick of the wrist of some obviously powerless, all-arms, half-assed “hit” and down goes Mr. Weightlifter. It just looks sloppy and stupid.
Well, that’s one problem; the other is that it’s becoming absurdly cliche and nothing new is being added to the “ass kicking chick” genre. Heck, it’s almost the only thing Joss Whedon knows to write about - if he ever gets his wish of doing a Wonder Woman movie I will bet $500 that Wonder Woman will mostly forego her other abilities for a lot of kung fu.
See also the Buffy TV series, where the NON-superpowered characters dispatch vampires by whacking them in the chest with a stake hard enough…to leave a small bruise. Maybe.
Driving a pointed piece of wood through a human breastbone is hard. Hence the stereotypical mallet to aid in the endeavor. It makes a vampires a heck of a lot less fearsome when they have an achilles chest that causes them to instantly disintegrate with what looks like a solid punch from a normal person. I don’t mind when Buffy does it because she has well-established supernatural strength - we’ve seen her punch a hole through a metal locker with her fist. But teensy little Willow? C’mon.
And again, I’m a fan of Whedon. But some of his ideas just cross the line into more-overt-than-usual for fantasy/sf tv show ridiculousness. Like horses outpacing RV’s and hovercrafts ( hovercrafts! ). Or hyena videos showing what are clearly Cape Hunting Dogs, NOT hyenas. Or cobramen with solenoglyphous fangs, when everyone knows that elapid snakes like cobras have proteroglyphous fangs. Or…umm…well…
Amusingly, she’s mentioned on that page as specifically NOT indulging in “She Fu”; rather, she uses Waif Fu. Note that she is literally used as the illustration for the latter.