IIRC, during production she drew the original with an even smaller head and was asked to go back and fix it by her production team. The final version still isn't quite right, but it's markedly improved over the first draft. This is a common problem, and doesn't really have anything to do with "formal training". Although her work is quite small in comparison, this happens often in larger works where the artist doesn't take adequate time to step away to make sure the proportions are all correct in correlation to each other. "Formal training" is a bunch of nonsense. All you need to do to become a better artist is draw from life, and do a hell of a lot of it. Looking at Kojima's work, it's obvious she's done a lot of drawing.