I really think more were saved during the Great Awakening, so that is hardly the greatest revival in history. But let's break it down. God sent them a prophet (albeit a reluctant one. Then what do we have? Let's look at the reaction to Jonah's message in ch. 3:The people of Nineveh, as God preached to them His prophecy of coming future judgement, and the Spirit then moved in biggest revival in history. the word of God was given forth, and the Spirit Himself did His task among them.
"7 And he caused it to be proclaimed and published through Nineveh by the decree of the king and his nobles, saying, Let neither man nor beast, herd nor flock, taste any thing: let them not feed, nor drink water: 8 But let man and beast be covered with sackcloth, and cry mightily unto God: yea, let them turn every one from his evil way, and from the violence that is in their hands."
It happened just as I said: individual repentance and corporate prayer. The people of Ninevah from the king on down fulfilled the conditions needed for revival.