Some increase in brain size does affect an animal’s capability for intelligence and behavior, many size differences only exist in a specific brain region. It’s mainly seen in animals that have more advanced senses, such as sight or hearing, exact movement. The authors suggest that “advanced” thinking requires a very limited number of nerve cells.
In conclusion, big animals aren’t as smart as the little ones. Big animals have big brains to move big parts; brain size has nothing to do with intelligence.
Source: http://www.livescience.com/animals/091124-big-brains-not-better.html