While humans have the largest brains proportional to the body weight of all animals, we don't have the biggest brains.
The sperm whale has the largest brain in the world. Incredibly, the sperm whale has a brain that weighs around 18 pounds. This fact is not that hard to believe considering that the sperm whale (or cachalot) is the most enormous toothed whale, not to mention the fact that it's the biggest toothed predator. The males are usually bigger than the females with an average length of a whopping 52 feet and an average weight of about 45 short tons. Females are slightly smaller with an average length of about 36 feet and an average weight of 15 short tons. Newborns are the smallest with a meager average length of 13 feet with their weights averaging approximately 1.1 short tons.
To put the size of the sperm whale's brain into perspective, it is at least five times as large as a human's brain. Their brains have a volume of approximately 8,000 cubic centimeters. Despite their large brain size, the sperm whales have a lower encephalization quotient compared to plenty of dolphin and whale species. This low quotient means that the sperm whale is not necessarily the smartest creature on the planet. In terms of relative and absolute size, the whale's cerebrum is unmatched in terms of size by any other creature on earth. The reduced olfactory system also translates to a poor sense of smell and taste. However, the auditory (hearing) system is inflated while the pyramidal tract is not that well developed.