Oh, you remind me my old games, when i conquered and destroyed 10+ civs early and thought nobody cant stop me. In fact I had really trouble to keep 7+ cities in prehistoric age and started to falling in reasearch because you need twice more cities to keep up research with AI on hardest difficulty. Overexpansion will kill your economy(and research) sooner than any AI

. On other side of world there are still peacefully developing AI which soon will have 8 - 10 cities. Just keep playing and you will see. When you survive to renaissance age on first positon and you will be tech leader than you win game.
Too bad AI cant handle too many cities in revolution option, their empires will soon crumble and decline when they reach 15-20 cities. Only few of them will handle it, but most end with 25 at max. Only once I saw AI with 45 cities( but at that time i had 200+ cities)