Throughout history, some rulers have been given the title "Great." For example,

Ramses II of Egypt is also known as "Ramses the Great." Why do you think a

ruler from ancient times would be given this title?

Because they are the ruler of their people, and many were thought to be descendants or friends with a higher power or god. since this person was obviously much higher in the social hierarchy because of this, they are "greater" than everyone else. sometimes they may have done a heroic act, too, that makes everyone believe in their bravery and courageousness.