honestly, i'm not an expert on that matter, but I think it depends more on the faction than on the beheaviour. as I experianced it, the roman have the habbits to send decisive full stack over an ennemy, so instead of 120467 small quater stack battle, you might have to fight 3 or 4 major main battle, then, when you get into their territories, some smaller one as they send their garrison at you... i'm playing mostly as Sweboz and those are the only example I have for big pitched battl;e, as my other neighbour the Kelts are rather a guerrilla type.

Well, this is talking by gaming experiance, and maybe it's not ''official'' but he! i hope i didn't confused you too much!