What constitutes an empire? What land area does a particular empire occupy? These are controversial subjects when pointing to a particular empire. In his work “Size and Duration of Empires: Growth-Decline Curves, 600 B.C. to 600 A.D.” Social Science History Vol. 3, Rein Taagepera has defined an empire as “any relatively large sovereign political entity whose components are not sovereign” and its size as the area over which the empire has some undisputed military and taxation prerogatives.
The British Empire that spread over 13.71 million square miles (35.5 million square kilometres) occupied approximately 23.84% of world land area.
The Mongol Empire, the second largest contiguous empire spread over 9.27 million square miles (24.0 million square kilometres) occupied approximately 16.11% of world land area. It was larger than the Russian Empire, Qing Dynasty and the Spanish Empire.
Genghis Khan (c. 1162 – August 18, 1227), founded the Mongol Empire, the second largest contiguous empire in history after his death. The Mongol Empire was only second to the British Empire (1920)