Thats an over simplification. Your numbers dosent take into account the terrain of a country just the total area/Population which isent a good if any indication of over population. look at a country like Japan which is mostly mountains that arent suited for anything but to look pretty. There you have a area/pop that would say there is still pleanty of room for more people but in reality there aren´t. The same goes for China. China has wast areas of mountains and desert not suitable to sustain human life.
Why China has 1.3 billion people is a question that is too complex for any forum nerd to answer by looking as wikipedia or alphawhatsitsname.