Hangzhou took the lead with 1 million people by 1200.
Located at the southern end of China's Grand Canal, Hangzhou was a major trading city and the capital for the second half of the Song dynasty.
Marco Polo, who visited in the late 13th century, called it "without doubt the finest and most splendid city in the world."
The Moroccan traveler Ibn Battuta, who visited in 1345, called it the largest city he had ever seen.
Hangzhou lost its capital status after the Song dynasty ended and gradually lost trade to more coastal cities.