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The two biggest populated countries of the world are China and India. Both are doing good in terms of growth with China leading and India trying to catch up. Moreover there are also cases where country is less populated like Switzerland but still one of most developed country of the world. I think population is not relevant, as long as you have plan to execute in right direction no one can stop you from going up.
This is what I am saying, a country that can make its population efficient, that population will become the wealth of that country. India is currently trying its best to utilize its population properly and they have been quite successful. To keep a country in balance, youth is needed for which the birth rate is an important factor, if the birth rate of a developed country continues to decrease regularly, then at some point the number of old people in that country will increase and the working youth will decrease. So population and birth rate are very important in determining the future of a country.