When people talk about the logistical limits on the size of medieval armies, are they primarily talking about limits to the number of men you can plausibly raise from a given area (so, with enough land, a medieval European state could have raised an Umayyad-sized army) or is it the difficulty of organizing the supply chain to feed everyone? If the latter, were the Umayyads simply better at requisitioning and managing the supplies than a medieval European-style state?