Head size and intellect?
 

Although I paint from photographic reference material, I don't feel any need to be confined by that "reality" - most especially when it comes to an anatomical proportion like head size.

When the subject has a large head, even though it looks "perfectly normal" in reality, this feature tends to make someone look a "wee bit stupid" in a portrait painting.

Below is an example of a portrait by John Singleton Copley in which the sitter's head size is probably accurate, but it doesn't really come across well.

And here is a portrait of King Philip IV by Velazquez. You can see that this subject has a verrrry small head in proportion to his elongated body.

Philip's small head size implies that he is an intelligent and worthy leader. Needless to say, it is oftentimes better to understate the size of a head.