I have way more female characters, especially in older stories (which I usually recycle into newer stories, so the casts remain lopsided). This is just because when I was younger, I felt there was a disturbing lack of female-led stories or interesting female characters in general, and I sought to fill that void single-handedly. ^^;
What's my present-day excuse, though? Well, I still think there aren't enough great female characters (spoiler alert: there will NEVER be enough) and also...due to the way gender norms are, they're just way more fun to design.
Every once in a while, when I try to do a mostly-male cast, I get SO SICK of drawing 8 million pairs of pants. Pants, pants, nothing but PANTS!! >_< Whoever decided that guys can only wear straight, featureless, boring clothes should be slapped...and that goes for the formal shoes, too. I think my current habit of designing guys with funky heels is less me being unique and more me not wanting to draw soulless loafers for the umpteenth time...
...Wow, I got off topic there. ^^; Anyway, that's really the only reason; I write guys and girls pretty much the same, so the only deciding factor is design.