Population size could be one factor. When you talk about India, each of its separate states are equal to or larger than the population size of many of the other countries on the list. It would also be interesting to see the breakdown of states where research is conducted in India. I'm sure you will find that some states are easier to do work (data accessibility, relative political/civil stability, etc). But I once thought the same thing about the amount of research that is done in India. I've come to the conclusion that since a large percentage of the world's population and a large percentage of the world's poor lives in India, its probably not too harmful to have so much research concentrated there. This of course disregards issues of external validity and generalizability, but I think population size could be a good justification.