Island dwarfism
One is based on the idea that a smaller territory means a smaller food supply. A process where only smaller animals trapped on the island survive, as food declines to a borderline level. Population declines and, as smaller animals need fewer resources and smaller territories, they will survive longer. Eventually food sources replenish, and the survivors flourish.  Smaller size is also advantageous from a reproductive standpoint, as it leads to shorter gestation periods and generation times.