I think it can be explained in mathematical terms by the doubling time formula. 70/r = n (doubling time) For instance, when r is 1 (one percent growth per year) a population will double in 70 years. China is an old culture and in there has been a lot of doubling. Currently their rate is .58% and population is 1,337,730,000. If that rate of growth hold, in 120 years, there will be nearly 2.7 billion Chinese.
The population of India is currently 1,181,348,000 but their rate of growth is 1.46%. That means there will be 3.6 billion Indians in only 44 years.