The Achaemenids enslaved conquered subjects and hoarded a substantial amount of wealth.
Ahsoka pre-conversion was remarkably brutal, but my point was about broader institutions, no necessarily enlightened rulers/tyrants. The Mauryas helped solidify the caste system, for instance.
As for whether there's a distinction between colonial empires and regular empires, that's a subject of debate.
I don't want to derail this thread, but I strongly disagree. Safety? Empires regularly dragged in their colonial periphery into conflicts that absolutely did not benefit them and were of a much larger magnitude than before, and many of these wars were also fought on the soil of neutral countries (various states of the 30 Years War HRE, WWI Iran, WWII Thailand). Plus, the empires themselves were frequently aggressors against natives.
Prosperity: Yeah, an empire occasionally did leave colonies better than their pre-colonial conditions, but that's assuming that pre-colonial societies stayed stagnant, of which there are a ton of examples showing otherwise. What improvements could an independent country make if its wealth and resources were not flowing to the metropole? And more often than not, it didn't even improve absolute metrics.
Total Population:
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Heights (indicative of nutrition):
Wages:
Except "grand" civilizations are often built on the backs of those who are oppressed. The late Roman Empire absolutely was oppressive. It's bureaucracy swelled, it conscripted soldiers, and it practiced slavery on a massive scale. A "simpler" society was also one that demanded less of its citizenry, especially in an era without democratic governance or substantial welfare.
Height indicators show that the average person in the Mediterranean was actually better fed in the "dark ages" than during the 3rd/4th century.