No other city would grow that large until the 19th century. People lived surprisingly long back then too — that is if you made it past the age of 5-10 years old. They say half the children died.
Pictured below is a visual representation of the boundaries of the Roman Empire from 510BC-1453AD.
Image By Sonse – Rome View from Piazza della Trinità dei Monti, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=76175888
Gif Roman_Empire_map.gif: Rokederivative work: Sémhur / CC BY-SA (http://creativecommons.org/licenses/by-sa/3.0/)