In the 1500's the largest city in the world was in China, and that is nothing weird when we take in account that China had the largest population in the world. But this was not to last for very long, as China was facing several centuries of stagnation and even regression, being ruled by bad emperors. The people didn't really had any motivation to move to the cities, so they remained in the rural areas where they were able to be engaged in agriculture. On the other side, in Europe, the UK was experiencing the opposite. The country had several centuries of excellent policies, which made it very prosperous, wealthy, and more powerful. In meantime, the Industrial revolution started there as well, and with the industry growing constantly, the people flocked into the cities to work in the factories instead of the farmlands. This resulted in an exponential growth of the cities, so the largest city in the world was now in the UK.