no actually you have to worry about overpopulation. the problem that comes with a super large population is not simply an amplified version of the problems a less populated society would face. there is a book written by scowcroft and brzezinsky, in it brzezinsky said that he went to China and had dinner with Jiang Zemin, and he asked him what is the biggest problem that China faces...out of all the possible choices you'd have like corruption, pollution, and wealth gap, Jiang said "too many people", and brzezinsky agreed (remember this guy is a geostrategist). marxist theory says that quantitative change up to a certain level induces qualitative change, this is the same thing, you cannot simply say "well this country have a similar pop. density so China should be just fine".
So when do you wish the government should act? untill the time when 30% of China's population were already over age 60??
Even if the government looses up its birth control by that time.. It would be hard to have your population increase immediately. Why? The more your country gets developed, the more life will be complicated, busier, stressful, or even harding to earn money to attain a certain standards of living.
It had already been proven in many developed nations, they tend to have lesser kids. Even in my country (Philippines) which is also a third world, rich people tend to have few kids only 1 or 2, while it was those poor people (belonging to the poverty line) who had as many as 12 kids at the age of 40.
When the time comes and China became a developed country, i don't think couples will still prefer to have many kids..
Are you getting my point? Example was our family, I belong to an upper class society, and I only had a sister. That's all.. were just 2. Get it?