The encephalization quotient is a number which is a quotient of brain size to body size. It is common wisdom that larger animals need larger brains. I find this a grossly simplistic reason. I agree that some animals need more complex brains as when evolution makes trade-offs of brain over brawn. This does not lead to a positive correlation however. The only simple correlations between body size and brain size that I see are:

• A larger body may have more muscles with need for independent control.
• A larger body may have more nerves to sense organs merely because a larger animal affords more sensory apparatus.