Do you really think so? I googled 1980 Chinese population, and came up with the following - China: 981m in 1980 and 1,338m in 2010 (36% increase) vs US: 226m in 1980 and 309m in 2010 (36% increase).
My impression is that the countries with the fastest population growth have tended to lag in GDP per capita growth, probably because the resources of the state are not infinite. India has a literacy rate of 74% (vs China's 100%), possibly because its education budget has lagged its much faster (than China) population growth. Pakistan's poor school-age children are educated in madrassas where they spend their days memorizing the Koran and the hadiths because they can't afford school fees at government-subsidized institutions.
Clearly education and other factors influence economic outcomes.
All other things being equal, demographics have a powerful independent effect on the economy.
The most economically stimulating element of the population are young adults, just starting out (that years graduating class). They need everything to furnish their households, will be raising expensive families, and have the energy to work long hours and second jobs. There had been a growing graduating class in China every year during the boom.
Additionally, China’s migration from the farms to the cities was the largest in human history - very stimulative.
Both of those powerful trends have peaked, and are now on a slowing trajectory.
The one child policy is an extreme intervention, whose effects are now pronounced in the age and gender distribution of China - it will take a generation to normalize, if allowed to do so.