Don't humans have bigger brains than all other
animals?
No. The brains of animals such as elephants and blue whales are
considerably larger than the brains of humans.
Most people therefore acknowledge that intelligence is not necessarily proportional to brain mass.
Nevertheless, many people will instead try to argue that intelligence is proportional to the ratio of brain mass to body mass, and will point out that humans have a much higher ratio of brain mass to body mass than do elephants or whales. However, the problem with this argument is that animals such as the pygmy shrew have a ratio of brain mass to body mass which is roughly equal to that of humans.
Follow up questions:
Don't only human beings have a soul?
How can some animal rights activists believe that other animals are our equals?