He was interviewed about this once:
Question: Why do you always choose a girl as your theme?
Miyazaki: I don't logically plan it that way. When we compare a man in action and a girl in action, I feel girls are more gallant. If a boy is walking with a long stride, I don't think anything particular, but if a girl is walking gallantly, I feel "that's cool." Maybe that's because I'm a man, and women may think it's cool when they see a young man striding. At first, I thought "this is no longer the era of men. This is no longer the era of taigimeibun." But after ten years, I grew tired of saying that. I just say "cause I like women." That has more reality.
The original quote was here: http://www.nausicaa.net/miyazaki/interviews/heroi
Cheers,
Paul
You're using a keyboard! How quaint!