LARGE kangaroos are unique among mammals in their ability to uncouple aerobic metabolic energy costs from the speed of locomotion(1,2), making hopping an economical gait. During the first half of the ground-contact phase, kinetic energy lost from the body is stored as elastic strain energy, predominantly in the hind limbs(3-5). The subsequent recoil returns kinetic and potential energy to the body. Here we show that the allometry of structures in the legs and feet of Macropodoidea is different from that of quadrupedal eutherian mammals. The potential for elastic energy storage in hoppers is shown to stale with strong positive allometry. This is a function of the structural properties of muscle-tendon units in the distal hind limbs and the postures adopted by hopping kangaroos. Our findings demonstrate how the use of tissue elasticity is strongly mass dependent and help explain the observed energetic phenomena.