Why there is less population in western countries?

It can be seen that, generally, the population is directly proportional to the number of poor people or being an undeveloped country. Only some countries like China is an exception to that. Why there is less population in Western countries and more in the Asian continent? Is it because of less education? Is it because sex safety measures are not properly followed by couples? Is it the fear of undergoing family planning operation? What is your take on it? Is it because of the lifestyle of people in Western countries? Why there is less population in western or in more developed countries?