Most of the newer research on this topic suggests that it's neural connection complexity, and specifically frontal lobe volume, rather than overall brain size that determines intelligence or brain power.
>Luckily, there is much more to a brain when you look at it under a microscope, and most neuroscientists now believe that the complexity of cellular and molecular organization of neural connections, or synapses, is what truly determines a brain’s computational capacity. This view is supported by findings that intelligence is more correlated with frontal lobe volume and volume of gray matter, which is dense in neural cell bodies and synapses, than sheer brain size. Other research comparing proteins at synapses between different species suggests that what makes up synapses at the molecular level has had a huge impact on intelligence throughout evolutionary history. So, although having a big brain is somewhat predictive of having big smarts, intelligence probably depends much more on how efficiently different parts of your brain communicate with each other.
If size of brains were any measure of IQ, elephants and whales would be the smartest animals on this planet.
Brains consume a lot of resources, so evolution had to optimize their efficiency. They need energy, they need cooling, they need removal of waste, they must deliver signals faster - all those things are pushing for miniaturization.
> We are not smarter than our ancestors. Human brain size is down 20% from peak, and is continuing to shrink.
Processors in 2000 were ~42nm in size, now they’re ~5nm in size. Not only did they get faster as they grew smaller, they became more efficient as well.
