It's one of those ideas that seems to make perfect sense: the bigger the brain, the more intelligent the creature. Exceptions are becoming increasingly common, yet the belief persists even among scientists. Most biologists, for example, assume that rats are smarter than mice. Scientists now challenge this belief. They compared mice and rats and found very similar levels of intelligence, a result that could have powerful implications for researchers studying complex behaviors and learning.

Source: In a battle of brains, bigger isn't...ice perform similarly in cognitive tests