Why do Asian nations have such a larger population than western nations? Has it always been this way? If not, when did it happen?

You are looking at the wrong meter. It´s not about large populations, but population density. China and India have huge populations, but China is actually lower than 80 if you rank the nations of the world by density. Germany, UK are higher. India is fairly dense, but still below 30th. Belgium and The Netherlands are a little denser, and Malta is a lot denser.

Asia is the biggest continent and with most countries. Some of them very dense, but they also have some of the least dense states, such as Mongolia and the Asian parts of Russia. Europe as a whole is similar to China in size, and probably equally dense below the Baltic sea. Imagine the EU as one state and its the third biggest state in the world.

There are a lot of factors that determine density of poplation. Climate, soil, culture, economic policies. But I dont think there is any Asia specific factor.