When resources are scarce in an area, the animals dealing with such scarcity will often evolve to be smaller, and thus use less resources. WebEcoist has a great post featuring 12 animals, including one human, that evolved into a dwarfish version of their original species.
My favorite is the dwarf elephant, not to be confused with the much larger pygmy elephant. These creatures were from the islands of Crete, Cyprus, Timor and Flores and those from Cyprus tipped the scales at only 440 pounds. Compare that to the average size of a savannah elephant that weighs over 4,000 pounds and you'll see just how tiny these little ones must have been.