Scientists have, "found a scaling law that connects population size to transport networks and economic activity and CO2 emissions. The relationship between these variables follows the same curve for all cities, large and small, they found, and echoes Kleiber’s Law, a principle that emerges in biology when the sizes of animals are viewed in relation to their metabolic rates.

"In the 1930s, Max Kleiber found that the energy that animals required daily to sustain their bodies (their metabolic rate) scaled in proportion to their size. Mice have a lower metabolic rate than elephants, and tiny microscopic creatures require less energy still. The law has proven a useful tool.