That can't be all though, can it? Big eyes are certainly common in anime and can be traced back decades in terms of cultural influence and the limitations of television animation and video game graphics. Her face is angular, her nose and mouth are very small. These are adult features. Even if we set aside the face, the original comment ignores everything else about the character. Ryza certainly has a youthful or spirited vibe but that's hardly enough to say she "looks like a child" given the proportions on the entire rest of her body.
I still don't understand what inspires this reoccurring statement on the art style, but I'm trying not to assume it's the obvious and horrible implication it places over the people who create it and enjoy it. That I never hear it thrown at anime men, just the adult women, hurts my optimism no small amount.