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.
Not many countries with such a large human population can catch up with what China has done. It takes advanced thinking from a leader to get their country out of the pressure of decline. China is trying to control and dominate the world economy through trade routes and the products they create and currently they almost control the Asian market. India is far behind compared to China, but they are slowly trying to implement a strategy like China, by offering products and industry to several Asian markets in particular.
Population is not very relevant which can prevent any country from getting out of economic problems, but a large population can also hinder the country's economic growth because a large population can make it difficult for the government to increase the source of income for the country. If the leader has a concept in managing the country then population size may not be very relevant which can affect the decline of the country.