[kayodelycaon]

Smaller animals are less massive, which makes the forces operating on them significantly less.

Our brains are too large and fragile and our fluid space is much thinner relative to our brains.

[dotancohen]

It's yet another application of the square-cube law. As the animal scales smaller, the frontal area of the brain goes down per the second power, while the mass goes down per the third power.