Rome took the lead in 200 AD with 1,200,000 citizens
From its humble roots as a small Italian village 1,100 years earlier, Rome in the second century AD was enjoying the pinnacle of its influence and achievement.
At this time, the city was a military dictatorship under Septimius Severus; A move the people welcomed to correct the corruption instilled by Emperor Commodus. Recall Joaquin Phoenix in "Gladiator" — that Commodus.
Rome reached this size because it could draw food and taxes from most of Europe and the Mediterranean, but it proved an untenable position. By 273 AD, Rome had fewer than 500,000 inhabitants and the Dark Ages were looming on the horizon.