A quick note regarding the premise of the question:

While China is the most populated country, it does not come close to being the most densely populated. Three countries that neighbor one another (and where all a part of British India)--India, Bangladesh, and Pakistan--all have far more densely gathered populations, and, despite the three countries total area adding up to less than half the total territory of China, the three nations' populations add up to more than that of China.