Charlemagne empire wasn't an interconnected fabric of roads, trade, centralized administration and so forth, ie. it wasn't all that that Roman Empire was, and all that having gone is the fall of Roman Empire. Charlemagne empire was just an aggregation of conquered lands under his personal rule which thus he easily divided between his sons.

Yes, but my point was that a large state was created and managed centrally for quite a long time, and even the successor states were large.

More than that, technology continued to evolve. The Romans had nothing like Medieval plate armor, for example. There are many examples of better tech from the supposed "Dark Ages".