Diana, not being in heavy armour, uh...makes sense for her to not use a big old choppy thing, even if she has the best reflexes in the group. That being said, she'd probably look just as good with the kopis, but I imagine they picked the xiphos because it was the more famous Greek sword, being the standard Spartan backup weapon and all. I think they were the only force to standardize gear there though. Most hoplite forces actually preferred to carry the kopis, or some other slashing blade, but that was FAR from standardized.
And lol about the breasts. Unless...of course, it's supposed to be metal armour, so it doesn't deform without its wearer...>_<