Well, recall that Rome eventually split up into two empires when it grew too large and its leadership was no longer effective. Once Alexander the great died, his empire was split up among his generals in part because none of his generals were charismatic enough to rule in Alexander's place. The historically large empires weren't large simply because they could conquer all their neighbours. Plenty of powerful, aggressive empires end up stalling their expansion and/or splitting up after a while; usually when a leader dies like in the case of Alexander the great. What allows an empire to grow truly massive and sustain their large size? Charismatic leadership for starters. A unified culture hegemony helps. In the end, though, the biggest factors tend to be trade and effective, organized governance. Finally, technology multiplies all these effects making it easier to have a large empire in later eras. Honestly, the most realistic mechanic that challenged expansive empires was the corruption mechanic a al Civ 2. Ancient empires truly couldn't expand to massive sizes in great part because border territories were far away from the emperor and the local leadership was far more loyal to enhancing their purse than furthering the emperor's goals. And, generally, there was nothing the emperor could do about it. But it seems the corruption mechanic is now considered a poor game mechanic that no one wants to return to. Growing Happiness and Health penalties per city is probably less realistic but it's a lot more interesting to play with.