Phyletic dwarfism
Phyletic dwarfism is the decrease in average size of animals of a species. There are a few circumstances that often lead to species doing this. Lack of predators of smaller creatures can allow smaller members of a species to survive. The lack of resources to sustain a large population of larger animals can pick off the largest specimens. Available resources being more beneficial for smaller creatures can also do so. These circumstances are common on islands making Insular dwarfism the most common form of phyletic dwarfism. Examples of this are the Channel Island fox, extinct dwarf elephants of Crete, and Brookesia micra, a minuscule chameleon from Madagascar. It is possible that phyletic dwarfism may have also helped into bringing us the birds we see today from the giant dinosaurs we read about as kids.