In an arctic environment, animals must do everything that is possible to conserve body heat. This is why no small mammals evolved in that kind of environment. Smaller animals have a very small interior body volume, thus they have a small surface to volume ratio. That means that a relatively larger percentage of their interior body volume is adjacent to their surface skin, thus allowing heat to escape their bodies more easily. Larger animals, on the other hand, have a larger surface to volume ratio. Much less of their body volume is adjacent to surface skin, so more of their body heat is naturally retained.