Brain size has more to do with body size than anything. Most of your brain functions as a motor control and sensory processing center. To willfully move muscle

Brain size has more to do with body size than anything. Most of your brain functions as a motor control and sensory processing center. To willfully move muscle in your body, requires brain power. This is why after a stroke, and a part of your brain dies, you can lose both the ability to move sections of your body, along with feeling anything in those sections. As so, larger animals have bigger bodies, and hence typically have bigger brains in order to handle/control their larger bodies.