[HorseSoldier]

Marauders are probably a lot like cults -- above a certain success level we start to call them religions, even if their beliefs remain kooky. Above a certain success level and size level, marauders will morph into (probably oppressive) local governments, though their predatory tactics may not change at all and the distinction may be wasted on their victims.

Probably the further away from organized military formations, the larger your potential marauder groups get. Close in to controlled areas, anything too threatening will get smashed, meaning you'll mostly see small units that dodge patrols and aren't enough of a problem to justify a dedicated campaign to wipe them out. Out in the middle of nowhere, there won't be that culling process for the most successful, so that's probably where you see large marauder bands turning into feudal aristocracy or whatever, or fighting it out with independent towns and settlements in bloody little knife fights for the crumbs of civilization.