In research published in Proceedings of the Royal Society B, an international group of scientists including Monash University's Dr Alistair Evans proposed a new theory explaining the diversity of mammal sizes -- from the Etruscan shrew which weighs around two grams, to the blue whale which clocks in at almost 200 tonnes. Surprisingly, baby weight relative to adult body mass is key.

Dr Evans, of the Monash School of Biological Sciences, said size impacts on all aspects of an animal's physiology and anatomy, and the roles it can play in ecosystems.

"Size is fundamental to your life and your body -- how fast your heart beats, how much food you need to eat, and how you move," Dr Evans said.

Following the extinction of the dinosaurs, mammals flourished and their size increased dramatically. The study examined the maximum size of groups including whales, elephants, primates and rodents over this period to examine the constraints on size.