According to some scientists, the size of the brain depends on three things - maternal nourishment, the size of the body, and complexity. Some scientists believe that the larger the animal, the smaller the brain-to-body size ratio. Some animals evolve larger sizes according to their needs. But it is not that larger animals will have large brain sizes. Brain size is not always a good indicator of intelligence. For example, humans and rodents have the same brain-to-body size ratio, but rodents are less intelligent than humans.