So looking at population trends across the World from one glance it is obvious that the advances brought by the Industrial Revolution was what made our population skyrocket and that pre-industrial populations either remained static or slightly growing.
But from seeing some population estimates in Wikipedia (not reliable, i know) and in internet forums it seems like cities like London had a population of 50,000 in 1400, then rising to 300,000 in 1600. Or Ming China having a population of ~60 million in the 15th century while rising to 100 million in about the end of the 16th century. Japan also having around 8 million in 15th century while jumping to 30 million at the start of the 18th century.
What caused these population spurts/increasing urbanization to occur in the Old World in the Early Modern era? Population just seemed flat during all of the Medieval era.