DH has a small head for a man.
I have a large head.

DD has a small head.
DS has a large head, larger than his older sister's in fact.

I don't know what to make of the data...

As for eugenics, I think it's a bad idea. Look at dog breeding. We selected dogs for what we thought were desireable traits but many breeds are now prone to problems specific to those breeds. Genetic diversity allows a species to be adaptable. We have no way of knowing which traits will be most valuable for humans in the future.