Power comes from the size of the empire you control. This is evident in geo-politics today. Russia’s Vladimir Putin, China’s Xi Jinping, and America’s Donald Trump hold the most political power because they control the largest territories.
At the height of his empire, Genghis Khan controlled an empire that spanned 11 million square-kilometers. The sheer size of this empire surpasses that of the empires of Rome, Alexander the Great, and Atilla the Hun. Ghengis Khan's military dominance at the time ensured victory over more organized and traditional armies. His soldiers were highly trained in mounted combat, which often decimated infantry units of opposing forces. When his armies took control over land, they often razed the land of resources and pillaged settlements.  Nothing stood in the way of his conquest until his death.
Most of Genghis Khan’s territory was uninhabited grassland. As a result, the size of his territory becomes almost meaningless. What use is holding a territory if there is nobody in it to exercise power over? The areas he controlled were mostly occupied by other nomadic tribes, which he simply gained control over. Very few cities and large settlements, and very little infrastructure were seized by Khan's armies. Additionally, he did not dictate a clear ideological or cultural doctrine to the lands he possessed. While some Mongol practices were adopted and implemented, local cultures largely went unchanged under Mongol administration. They were simply unified by an individual, but nothing deeper.
Having absolute power over your own tiny island makes you far less powerful than someone with only minor influence over a vast territory. Therefore, the size of the territory is a key component of evaluating power.
[P1] The most powerful person in history is the one with control over the largest territory. [P2] Genghis Kahn had absolute control over a vast territory. [P3] His military dominance meant that few were able to defeat him.
Rejecting the premises
[Rejecting P1] Most of the territory he controlled were rural villages and grasslands. [Rejecting P2] Genghis Khan did not greatly influence the lands he controlled. [Rejecting P3] Khan's influence did not extend past his military and territorial strength, as many regions largely maintained their local cultures.