So? Is he the greatest conqueror of all time? His empire was twice bigger then Roman Empire...
He did have the largest empire ever but a lot of the land he conquered was not vastly populated whereas the Roman and British empires usually were. He was still a great leader but I don't know about greatest.
That's because he killed the majority of the armies and peoples whom were drafted to protect their countries. Mongolia itself wasn't very big compared to China or some of the Middle Eastern countries yet he still managed to conquer those bigger countries.