I think the pinhead issue is cause by hot toys' sculptor trying to replicate this look. Perspective distortion makes the top of the head looks smaller/flatter. IMHO, the sculptor should've use more than just 1 image as the reference.
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The other problem is, hot toys want to reuse the old mouth plates. They forced the old mouth plates into the new head. It would be better for the sculptor to make from scratch than reusing old head.
Finally, to my knowledge, HT's sculptors are more traditional sculptors than digital sculptors. If you make the wrong proportions, digital sculptors can easily edit the file and test print as many as they like until they get the right proportions. Traditional sculptor, once they got the proportion wrong, they'll have to sculpt from scratch again. I think they know the head is too small but they couldn't do much about it as it would take more time to sculpt a new one.