In a perfect world, where access to technology and the wealth are equally distributed, the GDP of each country would be proportional to its population. In the real world that relationship exists on average (see the dot charts below) but with significant deviations from the trend on a GDP per capita basis. Many developed countries, being relatively less populous than their developing counterparts, have high levels of GDP, while the GDPs of less-developed countries, especially in Africa, are disproportionally low.
Will the enormous population growth in African countries strengthen their economies and the wealth of their citizens, or will this growth only give rise to even greater poverty? "The answer depends on how each country responds today with policies." - the World Bank, October 2015.
Sources: World Population Prospects: The 2015 Revision, IMF World Economic Outlook (WEO), October 2015, Barro and Lee Long-term Educational Attainment by Country (1870-2040)
Overview Africa in Focus: Population Prospects ◖Population Density◗ Demographics GDP per capita Education