Encephalization quotient
Encephalization quotient (EQ) or encephalization level is a relative brain size measure that is defined as the ratio between actual brain mass and predicted brain mass for an animal of a given size, which theorize approximate intelligence level or cognition of the species.
This is a more refined measurement than the raw brain-to-body mass ratio, as it takes into account allometric effects. The relationship, expressed as a formula, has been developed for mammals, and may not yield relevant results when applied outside this group.
Additionally to volume, mass or cell count, the energy expenditure of the brain could be compared with that of the rest of the body.
Brain size usually increases with body size in animals (is positively correlated), i.e. large animals usually have larger brains than smaller animals. The relationship is not linear, however. Generally, small mammals have relatively larger brains than big ones. Mice have a direct brain/body size ratio similar to humans (1/40), while elephants have a comparatively small brain/body size (1/560), despite being quite intelligent animals.
Several reasons for this trend are possible, one of which is that neural cells have a relative constant size. Some brain functions, like the brain pathway responsible for a basic task like drawing breath, are basically similar in a mouse and an elephant. Thus, the same amount of brain matter can govern breathing in a large or a small body. While not all control functions are independent of body size, some are, and hence large animals need comparatively less brain than small animals. This phenomenon has been called the cephalization factor: C = E ÷ S2 , where E and S are brain and body weights respectively, and C is the cephalization factor. To determine the formula for this factor, the brain/body weights of various mammals were plotted against each other, and the curve of E = C × S2 chosen as the best fit to that data.
...
Wikipedia