One child policy best suits to developing nations where population is increasing and no resources available to feed them. The income and expenditure of every family and if no scope for saving then family size bound to be small.
I am an Indian and one comment rightly pointed that large populous nations need it because inadequate resources and funds it is not possible could sustain such huge mass.In India family planning was introduced long back but religion and superstition is a factor that dominates the scene even ruling governemnt lost majority and power.. So family planning program remained stagnant but India's new middle class is increasingly practicing small family set up so in next two decades India's population growth will come down and it will stabilize .
The country to country situation differs.For your information. less population does not assure progress or development of the country, because in south America and part of Africa some nations are large by size with less population then they are poor and under authoritarian regimes.
Low and high population is not the bone of contention but to put them on job and and best way to turn them into civilian society is a challenge where nations across the globe are struggling.
Good question and thought provoking.