Runaway AI in general is a problem with this game. Agreed I've seen Persia do this a couple times myself (though other times they don't do so well, especially if a warmonger picks on them early on). Many of the AI bonuses benefit bigger empires more and smaller empires less, so the big get bigger and the smaller AI still bicker with each other as they get conquered. Their happiness bonuses on the middle difficulties, let alone the high difficulties, rarely even slows down their natural expansion. It would be better if the bonuses helped small empires more and had ceilings where the bonus would dissipate if the AI was doing well in an area. From a historical perspective, it makes sense, and puts the player in a David vs. Goliath position for gameplay, but there's no real in-game mechanism for David to win besides using some uncreative exploits and hoping Goliath randomly shakes out his units in an inefficient formation. From a gameplay perspective, it's good to have civs that are stronger, but not that much stronger, unless other real-world stuff like civil wars, rebellions from colonies, etc happened to break down civs that get too strong.