Surface area : Volume ratio
The smaller the organism, the greater the surface area: volume ratio (SA: VOL). The greater the SA: VOL ratio, the greater the heat loss from an organism, the greater the rate of gaseous exchange, and the greater the rate of water loss.
In cold environments, the large SA: VOL ratio of small organisms is a problem because they lose heat too easily. Therefore, usually, these organisms have a higher than average metabolic rate to produce enough energy to stay warm. Furthermore, these organisms try to reduce their surface area as much as possible by having short ears and tails with a lot of fur.
In hot environments, the small SA: VOL ration od large organisms is a problem because they heat up too much. Therefore, usually, these organisms have a lower than average metabolic rate to stay cool. Furthermore, these organisms try to increase their surface area as much as possible by having long ears and tails with little of fur.