Many, many species of land mammal display this miniaturization process when living on isolated islands. Even humans have done this in the past, with "Hobbit" people of Indonesia that were discover back about a decade ago. The last remaining mammoths in existence lived on isolated islands, and they had shrunk to the size of horses.So a mammal being on an island would be similar to a fish placed in a aquarium.
When I think of the Florida Key deer and the Deer in the region of Saskatchewan and compare the build of their body's the term Habitat Adaptation comes to mind. Interesting animals for sure.