It's the ratio of brain mass to body mass that matters, not simply head size. However, in the absense of non-lethal or invasive ways to measure brain mass, head surface area to body surface area is not a bad surrogate.
I was specifically talking about newborn babies. But you are right, though it is also important to include birth weight. Ray Peat talks about problems associating with low-birth weight babies like cerebral palsy and infant mortality.
But that wasn't really my point, if intelligence is going be determined, then it first has to be defined. You have Ray Peat referring to spiders as possibly being more intelligent than neurobiologist, therefore it can't be all about brain size (or according to this example, even having a brain). I don't deny the important of evolution or possibly having a future generation with large brains for better development and health. But according to the quote I posted, there has to be more than just larger brains. I have another quote.
A professor, at Oxford I think, was doing MRI studies of brains, and his grad students participated. He remarked that the lab's chess champion had such enlarged ventricles that his brain formed a layer inside his skull no thicker than the meat of a coconut. When I read that I remembered hearing that someone had commented to Stalin that "Molotov has a lot of brains," and Stalin said "yes, but they're stupid brains." The intellectual capacity of an ant or bee shows what a milligram or two of brain cells can do when they are well organized. Kurt Goldstein's view of organismic meaning, developed by working with brain-injured people, was applied to language by Merleau-Ponty; from a different tradition, Natalia Bekhtereva developed a very similar approach to self-actualizing therapies. The idea is that a larger meaning-pattern can elicit the necessary functions from living material."
-Ray Peat