A subcontinent is a large mass of land that is almost big enough to be a continent. Greenland is a subcontinent.
Greenland.
Possibly you are thinking of Greenland, which is a very large island, but not quite a continent.
Greenland
Greenland
Greenland
Greenland.
Greenland.
Greenland.
Australia is considered by some to be an island and a continent. It is the world's smallest continent, and it is an island because it is entirely surrounded by water. However, it is theoretically too large to be considered an island. The world's largest island is Greenland.
Greenland.
Greenland can not be it's own continent because it does not posess bio-geo diversity unique to it's own landmass as does say Australia which is itself a large island. Greenland shares much of its geology and biology with North America. Greenland is also part of the North American tectonic plate.