I don't think this requires a "social engineering" solution. When a population grows beyond what the resources available to it can sustain there will be famine and a large portion of that population will die, and eventually the population will reduce to a level where it can again be sustained by the resources available to it.

Additionally, larger populations offer more chance opportunities for new diseases to jump to humans from other organisms, and large concentrated populations are more susceptible to contagious pathogens than small or spread out populations. As global populations increase and metropolitan areas grow bigger and more dense we will see pandemics with increasing frequency, and we will eventually see "the big one" that takes out 10%+ of the world's population and reduces many urban populations by 40%+

Lastly as space and access to basic resources like water and arable land become increasingly scarce we will see wars with increasing frequency. Wars are great for controlling population because you not only kill off a lot of people, but you kill of men--killing men is great for slowing down a population's growth rate since you leave some fertile females with no breeding partner.

Those are the big ones that will keep us from ever exceeding our carrying population for more than a few generations.