There's two effects that the islands have on the size of the animals, reducing of size, or increasing of size. The reducing of size is known as island dwarfism, while the increase in size is known island gigantism.
The effect of the island environment effects different types of animals in different manner, and it also has to be taken in account the size of the island. In general, the small animals tend to increase their size on the islands, while the large animals tend to decrease in size. The reason for this is that the smaller animals, because of the isolation, usually lack predators or they are very few, but also have sufficient amounts of food, thus they grow in size. The larger animals though, decrease their size because there isn't enough food on the islands to support them, thus with the decrease in size they consume less. Also, since they usually lack predators, they do not have to be large in order to defend themselves.
