And yes, by that time Indian population will be like 20% of the world population. It should peak at about 10 billions in a 50-60 years. =)
India is a very small country, with only around 150 million hectares of land available for agriculture. If the population climbs to 2,000 million, then India will have to import most of its food from outside. That will slow down the economy even further. And when you compare the economic growth of India, you should do that against countries such as Sri Lanka and Pakistan, which are having similar economic conditions. You can't compare the economy of India against that of developed countries such as the US and Russia.