I've thought about your response, and I'm not an art expert but here is my take on the matter. I think when people are depicted as thoughtful, their eyes are normally drawn small. When the eyes are big, they seem to lack an inner insight and look more like they're reacting to the environment than self-reflecting. This is observable in human nature, from responses associated with fear, shock, and confusion (all emotions associated with external stimuli, and all eye-opening), compared to responses associated with thoughtful happiness, sadness, reflection, and sleep (all internally conducted, with little use of the eyes). Because her eyes are drawn huge, nothing is shown to suggest that something is formulating behind those eyes - it's all in the eyes. She's reacting to her environment. Her eye size has a second effect - the emotions that you see in her eyes are more suggestive of what she's thinking than her other facial features, simply because of the eye's size. Large eyes indicate fear or shock - her smile is contrary to these emotions, which goes against what the larger eyes are saying. This is an indication of her own self-conflict, but her larger eyes tell us what she's closer to thinking.