>>1005526You aren't entirely wrong in principle here, but in this specific case you are mistaken. Chevrotains are an example of primitive artiodactylans that never grew to large sizes. They are living fossils, animals that didn't change after the appearance of grasslands that prompted an explosion of all sizes and shapes of ruminants. They aren't even restricted to Asian islands but also found in Africa.
As for island dwarfism, that is something that does happen to large species when they get isolated on small landmasses. Lack of nutrients and space are the major reasons why smaller individuals are more succesful in those circumstances and the population starts to shift towards dwarfism. It doesn't happen to all species though. Small animals often experience island gigantism instead. Lack of predation may allow them to grow larger than usual, or if they are predators, the lack of competition allows them to become apex predators and grow to unusual sizes.
This is why the island of Flores once had dwarf elephants and humans but giant rats, varanids and storks. Of these the giant varanids are still around, Komodo dragons that is. It actually is an extant example of island gigantism, not dwarfism! While it is related to the giant Australian monitor Megalania, it is not its descendant.