NON-LINEAR. “The absolute number of neurons in the rodent and primate cerebral cortex does increase much faster in larger brains compared to the number of neurons in the combined brainstem, diencephalon and basal ganglia, and is accompanied by a similarly fast increase in the number of neurons in the cerebellum.”
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2776484/