Why are primate brains smarter than rodent brains of the same size?

Herculano-Houzel et al. ask whether a difference the cellular composition of rodent and primate brains might underlie the better cognitive abilities of primates. They show that that in primates:

...brain size increases approximately isometrically as a function of cell numbers, such that an 11x larger brain is built with 10x more neurons and {approx}12x more nonneuronal cells of relatively constant average size. This isometric function is in contrast to rodent brains, which increase faster in size than in numbers of neurons. As a consequence of the linear cellular scaling rules, primate brains have a larger number of neurons than rodent brains of similar size, presumably endowing them with greater computational power and cognitive abilities.

If the same rules relating numbers of neurons to brain size in rodents also applied to primates, a brain comparable to ours, with {approx}100 billion neurons, would weigh >45 kg and belong to a body of 109 tons, about the mass of the heaviest living mammal, the blue whale. This simple calculation indicates quite dramatically that cellular scaling rules differ between rodents and primates, not surprising given the different cognitive abilities of rodents and primates of similar brain size (e.g., between agoutis and owl monkeys or between capybaras and macaque monkeys).