Hmm... I think it's the head. I just try to follow the scale that I mostly saw on a drawing in this style. If I make the head a smaller it will make her proportion a little different and gonna make her looks more older.
I'm gonna use 3DCG as a comparison sample.
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If you try to shrink the head size of the left picture, you will got a proportion close to the middle or the right picture, depending how small you shrink the head, but in smaller scale version. That's my opinion.
But thanks for the input!
Err... I believe I already told it on my first post.
Yes, I'm using tablet. The software is PaintTool SAI since for me it makes a nice line art for manga style drawing.
Teach? Errr... I'm not good at it. Just keep drawing, see a lot of other people's drawings and learn about real human anatomy(although I'm still not good at this), it will help you at drawing.