Greetings!
Hello everyone! This is an interesting conversation! Lots of cool ideas and such!
Hmmm...now, Utrecht, you make some interesting comments...I'm certainly not an economist, but as you mentioned, my point #1 can account for it. However, I should point out to you that are you aware that in the first century AD, the Roman Empire dominated over 100 million people? In addition, at about the same time, the city of Rome had a population of over 1 million people? It's incredible, but quite true. The Middle Ages, and with the Dark Ages that followed the fall of the Roman Empire many things changed. Technology was forgotten, knowledge was lost, and two other huge things occured: The widespread, integrated economy of the empire was gone, and the foundation of political unity was gone. These things weren't really seen in Europe for over a thousand years after the fall of Rome. The Roman Army put 80,000 men into *one* battle--Cannae, in 216 BC--where they were slaughtered in one day.Soon, they were replaced with more armies. Rome was watered of course by incredible aquaducts, and fleets of ships from Egypt brought grain to Rome on a regular, timed schedule. The grain was stored in vast grain silos, and distributed to the populace along with free passes into the great Games to see gladiators die. It was all part of "bread and Circuses" that were provided the whole population of Rome. The Romans also used concrete, too. There have been recent archeological discoveries that Rome had developed the technology of concrete mixing, and in fact had concrete apartment buildings, with communal eating areas, throughout the city to provide affordable housing for the masses. There were also plentiful gymnasiums/bathhouses where it was every Roman's custom to go to at the end of the day, whether he was a wealthy Senator, or a rough stonemason. The city of Rome also had newspapers, and large public bulletin boards that were updated daily with news from the empire. Rome also had developed a "guild" if you will, of professional street-sweepers and maintenance men who worked for the government, and it was there job to travel throughout the city during all hours of the day, keeping it clean.
I mention all of this as testimony to the technology, advancement, wealth, and power that the Roman Empire had, *2000 years ago!*
Think about these achievements carefully for a moment.
Then, assume a world like it, but now with magic, spells, and all the other goodies from the D&D game. It is thus, from these extrapolations, that I find it quite attainable to have vast empires in the game that have vast wealth, huge armies, and many other refinements--beyond what our ancestors, the ancient Romans achieved--without such magical power!

Imagine what the Roman Empire would have been like with orders of wizards running around, and temples full of devout clerics!
Neat stuff, huh?
Semper Fidelis,
SHARK