I don't always take the time to cite refs, but I should have been more clear I was talking about elephant and whale brains as being larger in neuron counts.

"We are probably near some asymptotic limit of brain size. In three very separate lineages - elephant, whale and hominid - brains reached a limit around 200 billion neurons or so and then petered out."

Ever since early tool use and proto-language, scaling up the brain was advantageous for our hominid ancestors, and it in some sense even overscaled, such that we have birthing issues.

For big animals like elephants and whales especially, the costs for larger brains are very low. So the key question is then why aren't their brains bigger? Trillions of neurons would have almost no extra cost for a 100 ton monster like a blue whale, which is already the size of a hippo at birth.

But instead a blue whale just has order 10^11 neurons, just like us or elephants, even though its brain only amounts to a minuscule 0.007% of its mass. The reasonable explanation: there is no advantage to further scaling - perhaps latency? Or more likely, that there are limits of what you can do with one set of largely serial IO interfaces. These are quick theories - I'm not claiming to know why - just that its interesting.