They are similar in size only in the flat map ( because projecting the globe into rectangle ( Mercator Projection ) has its toll in size of the land masses represented especially near the poles ) but actually speaking Australia is about 3.5 times larger and lies on a separate bed rock unlike Greenland which is connected to the bed rock of North America. Also there isn't a clear definition of what a continent must be in terms of its size or any other factors.
