Tokyo might possibly have been the worlds largest city since the eighteenth century

 "In the seventeenth & eighteenth centuries Japan was one of the most highly urbanized countries in the world. Edo (present-day Tokyo), with a million inhabitants, was probably the largest city in the eighteenth century world" page 11

 Source: The Japanese Consumer - An alternative Economic History of Modern Japan by Penelope Francks, 2009. Cambridge University Press, ISBN: 978-0-521-87596-7, page 11