Short answer: Science, bitch!
She lives in a brutal world and grew up on the battlefield. Her personal history is full of death, murder and cruel things (I don't want to spoil things, but she experienced a lot of fucked up stuff). These experiences do not only influence our mental health but also the physical appearance.
Her face is not this kind of expression-bling-bling, big eye shojo-glitter template because she has to survive on the battlefield everyday (see this image of a soldier in afghanistan). This is war and not a romantic sightseeing tour in pokemon safari zone. You're most likely not happy about that fact and your facial features will become more and more harsh.
Exposed to violence daily the testosterone level most likey increases - it will be higher than normal even for women. That's why the development of typical female attributes (huge breasts, curvy lines, big eyes) will at least be stunted. This leads to a more male phenotype.
And when I designed her, I tried to imagine how she feels, what problems she has and incorporate all her insecurities, fears and emotions in her design. She has a dark past and this past reflects on her design and how she behaves.
To conclude, I also wanted to contrast to all that badass, sexualized females in other comics, mangas and movies. Deal with it! It's not a shojo or shonen manga where the size of her boobies and eyes correlate to her fighting strength!