"Other studies since then have confirmed the finding: For every excess pound put on the body, the brain gets a little smaller. How ironic that the bigger the body gets, the smaller the brain gets."
Perlmutter, D. (2013) Grain Brain. New York, NY: Little, Brown and Company. Page 121, Hardcover.
Image of a walnut: iPhone5s. Do you know what animal is said to have had a brain the size of a walnut? (Answer)