The sheer mass of people that India and China has. If they wanted to, they could send a fraction of their population to pretty much any country and double the head count. It would be a pretty effective method to invade peacefully.
Nigeria won't have 800 million people by 2100, nor Africa 4.4 billion. There'll be some catastrophe well before then.
China is already doing that
India's started doing it too. The situation with Indian foreign students in Australia/NZ right now closely resembles that with Chinese ones in the 1990's, so I predict the high immigration that happened with Chinese then will happen with Indians soon.