It takes 24 million generations for a mouse-sized animal to evolve into an elephant-sized animal, but only two million generations to go the other way.
Scientists discovered this by comparing the body masses of nine different lineages of mammal evolution over 70 million years.
While being large is advantageous for regulating body temperatures and avoiding predation, scientists believe competition for food may drive the selection pressures to shrink.
Read more at Monash University