The Sperm whale has the biggest brain of any animal species, weighing up to 20 pounds (7 to 9 kilograms). But larger brains don't necessarily make a smarter mammal. As mammals evolved, many groups, from Primates to Carnivora, have shown independent increases in brain size. The fact that unusually big brains show up again and again in mammalian evolution suggests that bigger brains must provide some major benefits.

Source: American Museum of Natural History