According to Brazilian neuroscientist Suzana Herculano-Houzel the reason humans are so smart is not because humans have the biggest brains for our body size per se, (highest encephalization quotient) but rather because we have both a big brain and a primate brain. SHH’s theory is that primate brains are especially densely packed with neurons, so even though our brains are small compared to whales and elephants, the primate packing density gives us more cerebral neurons.
Well King Kong has a far bigger brain than humans do, and King Kong is also a primate, so by SHH’s logic, he should be smarter than people. And yet King Kong seemed no smarter than your average gorilla, which makes sense because King Kong’s huge body guarantees it would have a much smaller encephalization quotient than people do.
So while cerebral neuron count might be roughly as accurate as EQ when predicting the intelligence of real life species, it fails when it comes to fiction.