http://www.psych.ufl.edu/~steh/PSB6099/brainnecessary.pdf
Due to hydrocephalus his brain was pressed against his skull and only a mm thick. Fascinating read.
It appears the amount of white matter in the brain can vary wildly without having a large effect on cognition (as with the 126 IQ example). Usually it's the cerebral cortex, the outer, wrinkled brain layer that matters most. Humans have more neurons in this region than any other mammal by far.
This squares with the original article, which argues the absolute number of neurons in the cerebral cortex is more accurate for measuring cognition than sheer brain size. So we might derive that brain mass lost due to hydrocephaly is not nearly as important as the cortex neurons retained.
Except dolphins, of course. https://en.wikipedia.org/wiki/List_of_animals_by_number_of_n...
It adds a wrinkle, but appears to be larger due to brain volume rather than density:
However, as neuron density in long-finned pilot whales is lower than that in humans, their higher cell number appears to be due to their larger brain. .... Our results underscore that correlations between cognitive performance and absolute neocortical neuron numbers across animal orders or classes are of limited value - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4244864/
Which I take to mean we shouldn't rely on one indicator to explain the emergence of consciousness. It used to be thought that brain size dictated intelligence. Corvid tool is one challenge to that assumption. We shouldn't replace brain size with neuron size. It looks like evolution fleshed out multiple paths to achieving certain reasoning behaviors.