Geography question

Because a continent is not defined only by its large size, but also by the faulting system surrounding it. Greenland is not so well separated from North America, and it is tied to the North-American continent by its evolution. Australia on the other hand is a crustal plate well delimited from the surrounding plates, and it has a very own evolution. Australia is more linked to the African and Antarctic plates (more distant) than to Asia.

Thanks to staville for the response as I didn't realise that either. We (in Australia) are always taught at school that Australia is the smallest continent on the earth and the largest island...