The fact that mammal brains reached similar upper neuron counts (200-100 billion neurons) in three separate unrelated lineages with widely varying body sizes to me is a strong hint of an asymptotic limit.

Also, Neanderthals had significantly larger brains and perhaps twice as many neurons (just a guess based on size) - and yet they were out-competed by smaller brained homo-sapiens.

The bottle-nose could grow to the size of the orca, but its not clear at all that its brain would grow over a few hundred billion neurons.

The biggest whale brains are several times heavier than human or elephant brains, but the extra mass is glial cells, not neurons.

And if you look at how the brain actually works, a size limit makes perfect sense due to wiring constraints and signal propagation delays mentioned earlier.