I don't think space is really such a factor at the scale of a single city in many cases. Although China has a lot more land should it choose to build new cities or whatever, what matters more for existing cities is land in close proximity to them (to the point where transportation becomes the limiting factor).

Tokyo, for instance, has plenty of relatively empty space around it even today; it isn't severely geographically constrained in the same way that Hong Kong or Manhattan are.