The Economist explains the discrepancy between China's population today with what it would be if the one-child policy had been strictly enforced:
If each woman had been allowed only one child since 1980, China’s population would have been 340m smaller than it was in 2010. If a strict one-child limit were in force for the rest of this century China’s population would shrink to less than 145m by 2100, 800m fewer than the UN projects in its central scenario.
The Dish 