In one vh game (I was Portugal) Hungary managed to grab a ~25 provinces empire. Yes, it was huge: Venice to Caesarea, Corinthe to Hamburg. Ok, then the Pope was pretty angry at them, and two crusades and after every neighbour faction declared war against them their empire was pretty much shattered. But then the Pope had a nice empire from Genua to Naples and from Ragusa to Caesarea
Sorry for the city names, I tried to translate, but I also guessed a lot. Hope you find out which cities I mean

Ok, you are right: on vh huge empires are rarely seen. But I think it is ok this way: It did not happen in history so why should the game support the AI to build such huge empires? It is hard enough on vh because everyone around you really hates you (unless you have a very chivalric leader and pay good sums to your neighbours). You don´t need a 30 provinces Venice to threaten you.