Around 1 AD the population of the world may have been 200 million — but if so, a staggering one third *or more* lived in the Roman Empire. My guess is, population estimates for the past are probably too low; also, the family size went up in the (mostly riverine) agricultural civilizations, and infant mortality went down. Both were also terribly high compared to now.
And yet that could be probable that 1/3rd or more lived in the roman Empire. The Roman Empire at that time covered two of the 5 big culture agglomerations (the others being the ganges valley of india, china’s Han civilisation and the Mayas) and wasn’t hit by internecine wars as in China.