It is estimated that in 50 BC, the area that is now France had a population of about 2.5 million. At the same time, India was estimated to already have a population of about 100 million. By 1600, the French population had increased to about 20 million, while the Indian population was still about 100 million. Why the difference?
Of course we can't know for sure, but the most plausible reason (for me) was that both populations were at approximately the Malthusian limits imposed by their technology at the time, and that French food production, social arrangements, sanitation, and public safety were both very primitive compared to India in 50 BC, but had largely caught up by 1600. Meanwhile, globally, little progress had been made beyond the Indian technology of 50 BC. By 1800, the French had added another 45% (to 29 million) to their population and India had added perhaps as much as 100% (to 200 million),(all numbers before 1800 are estimates). By this point, technological diffusion had probably shifted to be predominantly from the West to the East, in contrast to the earlier period, reflecting the scientific and revolution and the beginnings of its industrial counterpart.
From 1800 to 1947 India's population grew to 360 million (80 %) while that of France grew only 34%, to 40 million. By this point, it is clear that both reproductive choices and production technology were important factors, at least in France. (Reproductive choices probably played a role in earlier times too, but data is very limited). Emigration had also become an important factor, especially for Europe.
After independence, India's population grew very rapidly to the present (330%), probably reflecting both economic factors (escape from the expense of supporting colonial rulers) and the rapid progress in medicine, etc., etc. France saw a smaller but still substantial growth of not quite 60%, while Kenya added an astonishing 800%. I only have economic data from 1960 to present, but during that period Kenya's GDP has grown 10 fold, France's 4 fold, and India's 12 fold.
Comparing these numbers to the demographic growth shows quite clearly why rich France has gotten richer, poor India has become an emerging (but still poor) powerhouse, and Kenya, which was a good deal richer than China or India in 1960, has now fallen well behind them. And then there is China: population growth 250%, GDP growth 70 fold. The average Chinese today is only slightly poorer than the average Frenchman was in 1960, whereas in 1960 he had less than 1/15 that much.