Siegmeyer is hardly the post powerful character in DkS, though x) And even as I agree with you, I'd like to argue that male and female armors are not to ne designed the same way because the bodies that will wear them will be different. Men are (post of the times, naturally) stronger and heavier while women are more flexible and so. There are anatomic differences between the two of them.

Sometimes, it dors not have any impact on the final armor, because the fighting style requires common qualities for the two genders (for exemple Siegmeyer and his daughter's armor are the same because they have the exact same style), but sometimes, given that there are things men can do that most women can't do (and the other way around), the armors need to be different.

I had the idea that female DRG try to avoid and hit while male DRG try to endure and hit, given the high level of flexibility and agility that one has to have to master the DRG style, which may advantage women when it comes to avoidance. That explains why men have chainmails while women have not : It reduces the flexibility but enables to endure more violent hits.

In FFXIV, there's little to no difference between male and female gear (lvl 1 craft gear excepted), and the only big differences can be explaind with the lore, as I did with the DRG. I'm sure there will be no difference between male and female DRK AF, and if there is one, that it will be properly justified in terms of fighting style.