There are lots of manly figures and monsters and robots, but there's no doubt "pretty girl" figures are the strongest sellers.
With newer anime - the kind most figures are coming from - female characters tend to outnumber male characters, and they're usually more eye-catching and interesting than the males too. And most of these shows are targeted at guys to begin with. Even if every character got a figure, teenagers girls would be the most common by far
Girl figures sell well to both males and females. There's definitely some 3D-2D fan love going on though. Did you hear about the Asahina Mikuru bunny outfit figure? It has squishable breasts, and even more amazingly, the box says they might come off if you squish them too hard.
As for why the girls tend to be either petite or really busty - I guess either extreme is more attractive to that fanbase than a "normal" looking woman. Unless it's Servant Saber. Most guys, otaku or not, have a type.
ZenErik loves red-haired girls and I like girls with swords. Those types are pretty common for this audience.
It all comes down to the anime and merch industries zeroing in on the reliable, loyal, male fanbase by repeating things that have sold well in the past.