For most of human prehistory, humans were hunter-gatherers. This means they were semi-nomadic and gathered plants and killed animals for food. It was a pleasant life that involved much free time and little work, but it involved more physical exercise and, being nomadic, mothers would carry their offspring a lot, plus eat a diet lower in fat and higher in carbohydrates, and they’d breastfeed for 3-4 years for each kid. This lifestyle meant that women tended to be a lot less fertile than now and had a child only around one every 4 years, with a tots of maybe 7 children in 30 or so years of fertility max, despite no birth control. With higher infant mortality, this meant that human populations didn’t grow very much and population stayed fairly steady and low for tens of thousands of years.
Then, agriculture was invented. Now, people weren’t so nomadic. They stayed in the same place to grow crops. Their diet changed a lot. Less meat, less protein, less fat, and carbohydrates in the form of grain became the main food source. This change in lifestyle meant that women became more fertile, and could have a child every few years. Population grew rapidly in areas where there was agriculture, totally crowding out hunter-gatherers with their nomadic, low populations. So the earlier agricultural was developed in an area, the faster that population grew. Agriculture was adopted fairly early in Asia and, most key to the large population there, is that they had a combination of fertile soil and rice, a grain that could be produced in large quantities on small patches of land—more than efficiently than grains in other areas of the world. So Asian populations grew as fast as humanly possible until the carrying capacity of the land was reached. Europe and North Africa also had agriculture but the carrying capacity of the land was lower so population stayed lower overall.