> as conquring the whole world

Unless it's a matter of speech, he conquered about 3.5% of the world's land mass and about a third of the (estimated) population, neither of which put you in the top 3 and couldn't be considered the 'whole world' either. By land mass it's not even in the top 20 largest empires ever. He was unique not for the greatest empire but rather for being so primely responsible for the massive empire he did create and in doing it all in about 12 years, the British empire for example by comparison has no single persons who were so primely responsible with many leaders emerging over hundreds of years, combined with his personal role in battles, all of which ending in victory, makes him the stuff of legends.