Regardless of what one thinks about the policy itself, in terms of achieving manageable population growth, the Chinese 1-child policy has been extremely successful. China's population growth rate has slowed dramatically over the past 30 years or so. It's quite possible that within 10 years or so India will have a larger population than China.
The population should half with this policy. If you, for example have forty people, twenty men and twenty woman and they get married and have one baby, that makes twenty children. If people somehow manage to only have boys, or girls for that matter, the next lot of babies will be a good deal less.
People live quite a lot longer than their reproductive age though. Twenty people having ten babies means a population growth of 50%, rather than a reduction of 50%. It takes a while for the benefits to kick in.
What's up with this glass? Excuse me? Excuse me? This is my glass? I don't think so. My glass was full! And it was a bigger glass!