A recent study showed that survivors had brains that were 53% larger, which was perhaps useful in avoiding predators. The problem is, it is genuinely unclear what role brain size plays in intelligence:
According to one study, lemurs, with brains 1/200th the size of chimps’ brains, passed the same IQ test.
Sometimes a small brain is actually an advantage. This is especially true of flighted life forms like birds and insects. Consider the gnat ogre fly, with a brain “smaller than the period at the end of this sentence”: “The researchers attribute the fly’s ability to adjust its trajectory so rapidly to its small size, which allows signals to travel rapidly from eye to brain to flight muscles.”
Small brains can be highly organized for survival: “ … researchers say, fly brains may be organized so as to make predictions based on universal design aspects of animal nervous systems, to avoid the swat.” Thus, the fact that the fly has only 100,000 to a million neurons depending on species, and you have 86 billion doesn’t improve your chances of a successful swat because the fly’s neurons are organized with only a few goals (like anti-swat), not thousands of them (as you have).
Of course, it is possible that the Quaternary megafaunas’ brains were not particularly well organized but that would be a challenge to study with the materials we currently have.News, “Did small brains doom the mammoth and the giant armadillo?” at Mind Matters News (May 20, 2022)
Takehome: Before we decide, let us hear a word in defense of small brains. The topic is not as simple as many think.
You may also wish to read: Can largely rearranged genomes explain why octopuses are smart? Even compared to each other, the genomes of three cephalopods studied had been broken up and extensively reorganized. The relationship between massive genome rearrangement and very high intelligence in an invertebrate remains unclear but it is a promising research avenue.