Metabolic activity sets the rates of individual resource uptake from the environment and resource allocations. For this reason, relationship with body size has been heavily documented from ecosystems to cells. Until now, most of the studies used the fluxes of oxygen as proxy of energy output without knowledge of the efficiency of biological systems to convert oxygen into ATP. The aim of this study was to examine the allometry of coupling efficiency (ATP/O) of skeletal muscle mitochondria isolated from twelve mammal species ranging from 6 g to 550 kg. Mitochondrial efficiencies were measured at different steady states of phosphorylation. The efficiencies increased sharply at higher metabolic rates. We have shown that body mass dependence of mitochondrial efficiency depends on metabolic intensity in skeletal muscles of mammals. Mitochondrial efficiency positively depends on body mass when mitochondria are close to the basal metabolic rate, however the efficiency is independent of body mass at the maximum metabolic rate. As a result, it follows that large mammals exhibit a faster dynamic increase in ATP/O than small species when mitochondria shift from basal to maximal activities. Finally, the invariant value of maximal coupling efficiency across mammal species could partly explain why scaling exponent values are very close to 1 at maximal metabolic rates.