What phenomenon explains that bulky animals are more easily able to maintain a constant high body temperature?

Gigantothermy (sometimes called ectothermic homeothermy or inertial homeothermy) is a phenomenon with significance in biology and paleontology, whereby large, bulky ectothermic animals are more easily able to maintain a constant, relatively high body temperature than smaller animals by virtue of their smaller surface area to volume ratio. A bigger animal has proportionately less of its body close to the outside environment than a smaller animal of otherwise similar shape, and so it gains heat from, or loses heat to, the environment much more slowly.

The phenomenon is important in the biology of ectothermic megafauna, such as large turtles, and aquatic reptiles like ichthyosaurs and mosasaurs. Gigantotherms, though almost always ectothermic, generally have a body temperature similar to that of endotherms. It has been suggested that the larger dinosaurs would have been gigantothermic, rendering them virtually homeothermic.

Gigantothermy allows animals to maintain body temperature, but is most likely detrimental to endurance and muscle power as compared with endotherms. Mammals' bodies have roughly four times as much surface area occupied by mitochondria as reptiles, necessitating larger energy demands, and consequently producing more heat to use in thermoregulation. An ectotherm the same size of an endotherm would not be able to remain as active as the endotherm, as heat is modulated behaviorally rather than biochemically. More time is dedicated to basking than eating.