It's common in martial arts demonstrations for the instructor to pick out a big guy to prove their skill. Sometimes the instructor will strike and hit big guys who stand their and do nothing, sometimes they'll lock or throw them. It's not impressive imo to toss around or strike a compliant uke. It's impressive to the untrained audience but that's it. You're right though, a diminutive woman will probably never be able to naturally build an intimidating physique.I think that all depends upon what a martial artist is trying to accomplish.
I initially began studying aikido because a very petite 90 pound woman (Lynn Fabia, who was then senior student under Bill Sosa) tossed me bodily onto my face during a seminar. I had done some martial arts training and grew up in a bad part of town so had quite a bit of experience with fighting. At the time, I was 6' tall and weighed about 210. She picked me out for the demonstration at the beginning of the seminar because I was so much larger than she was, and in pretty good shape. She then proceeded to slam me into the mat. Not just once, but several times. I was suitably impressed, and had to start learning. I seriously doubt that lifting weights and getting bigger and stronger would have done her a bit of good. However, her skills as a martial artist were incredibly impressive.
Its really just a thought on my part that training time may be better spent lifting weights and getting bigger. I'm a male and was thinking about myself in this example. If I'd have spent the time I do training focusing on getting big I'd probably impress a lot more people. Not that everyone wants to be big, I dont.