Without a question, China and India are the 2 largest countries in the world, population wise.  And while they started at a disadvantage in the race to modernization, both have grabbed for the brass ring with a gusto.  India led, but China is coming up fast (and is believed by many to be the prime reason for high oil prices around the world). 

But while China has instituted one of the harshest birth control programs in the world, India has not really instituted a state wide program.  And so many believe that in this century, India will pass China as the most populated country on the planet.

But there are factors now indicating that each country is heading for a Population crash in the near future.  We are not talking about a birth rate like that of Western Europe.  At least not yet.  But that could be only a matter of time.  For in both countries, women are choosing to abort girl babies at a much higher rate than boy babies.  This is being done for different reasons in the 2 countries, but the end result will be the same.  An over abundance of men, with no women to marry.  And while one man can impregnate many women, and indeed in many Muslim countries, men are permitted to have up to 4 wifes, the simple biological fact is that women can only be pregnant once at a time (sorry that that twisted sentence, but I think you get my gist).

And it is unlikely, due to the cultural of these 2 countries, that men are going to be willing to share their wives.  So in a way, China's policy will work, albeit not the way they had envisioned it.  And India's lack of a policy will work, due to their culture.  So probably in the 22nd century, the 2 most populous nations on the earth will start to decline in population.  I dont think either are in any foreseeable danger of disappearing like some Europeans nations, but the multi-billions will shrink to hundreds of millions.  And they will join the West in the looming crises of supporting an aging population (Japan has already joined the west in that regard).

With all the screaming of the environmentalist, and the closet eugenics of some abortion proponents, the real controlling factor of the planet will be mother nature itself.  For deny it as much as we like, man is a part of nature.  And Nature has been taking care of itself long before man crawled out of the sea, or monkeys out of the trees.  For while we can say these changes are cultural, culture is a reaction of man to his environment.  And his inherent drive to maintain the species.  It is clear that China and India could not keep growing with no end in sight.  But through their own ways, as individuals, not as a collective society, the ends are being accomplished.