OP's concern is very legitimate. Just look at EU4 Timurids and Ming, or Byzantium in CK2.
They did finally weaken the ERE in CK2 (after 5 years) by adding in the new imperial succession law. Byzantium does have tons of problems now like it did in real life, and even collapses or loses territory.
They also weakened Timmy and Ming. Well actually I can't recall Timurids being a threat ever, they tended to just implode before. Now the whole setup is different and there's no clear winner in the region. You often see Transoxiana victorious, for example. Ming are shackled by their Government form and the AI tends to want to make its neighbours Tributaries leaving it with limited scope to actually expand.
The Ottomans have even been balanced a little by a stronger Mamluks and fewer free cores. France can blob out if all goes well but it starts in a position that is precarious for an AI or even just a competent but inexperienced human.
I guess the issue here is that if you make some countries like Maurya as strong in the start as they historically were, then they'll just grow and grow - but will be there be something that then causes them to implode? Maybe the game's rebel system will be good enough to see large AIs sometimes lose a chunk of territory to separatists. The problem tends to be that these games are all about how empires form and expand, but not so much about how they cease to be. So it would be great if there were mechanics that made keeping a large empire together somewhat challenging. That would mean that human players wouldn't get bored once there were no longer any external threats, and AI that blobbed a lot would be likely to collapse dynamically.
