This has probably got to do with the fact that in M:TW all you had to do was move an army into a region to have a battle while in R:TW you can gain time by simply harrasing full stacks.
In my England campaign the Timurids, Mongols and Hungary have carved out empires, while the Moors, Russia and Denmark are still kicking on with one province, and Poland has only its king in a fort.
By the end game only the factions that are quite distant from you have empires.
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