I don't know what others' experience is, but blitzing other nearby factions constantly, from early to end game, makes any difficulty feel just the same, regardless of map size. Enemies may build tons armies, but they are weak against my 'charge' + 'initiative' boosted cavalry (horse/warg), regardless of champions or drakes/dragons on my side. I just blitz them all with my units with the charge talent and initiative. Nothing could stop me rolling them over. Game's usually over when I'm only half way through the warfare tech tree, and not even there for the others.
I wonder if there's anything the AI could do to counter my rapid expansion and conquests?