Asked by: Chris Sant Cassia, Malta
There are two reasons. First, brain structure is more important than brain size, and human brains with their highly folded and complicated cortex can do things no other brains can.
Second, relative size is more important than absolute size. The ratio of brain weight to body weight is about 1:5000 for fish, 1:220 for birds and 1:180 for mammals, with the most intelligent species generally having the highest ratio. For example magpies and crows have a higher ratio than most birds, and social mammals such as chimpanzees and dolphins have a higher ratio than other mammals. The human ratio is highest at about 1:50.
The largest brain of all belongs to the sperm whale, but dolphins have a higher ratio and are capable of imitation and can recognise themselves in a mirror. What they think of human intelligence we do not know.