I think perhaps you're conflating world population with the Chinese population. Overpopulation can occur in small areas without the world being too overpopulated as a whole -- most of them in SE Asia. If theres lots of people and limited resources, the math simply won't hold up. It affects issues like the environment and human health.
Singapore is very developed. Overpopulation is more a problem for densely populated developing places - Bangladesh, Victorian London, South American shanty towns et cetera.
