Phyletic dwarfism is the decrease in average size of animals of a species.
The lack of resources to sustain a large population of larger animals can pick off the largest specimens.
Examples of this are the Channel Island fox, extinct dwarf elephants of Crete, and Brookesia micra, a minuscule chameleon from Madagascar.
An noninsular example is the evolution of dwarfed marmosets and tamarins among New World monkeys.
[1] Phyletic dwarfism may have also helped give rise to birds from their much larger dinosaur ancestors.