A well educated country with sizeable population would have a high purchasing power consequently the economy of that country would improve, China with their huge population had well utilized it, they have a large strength of military personnel which is a very big advantage, in technology many of their youths are trained to build and assembled electronic, electrical gadgets and other equipment and so on, though the country has put on policy to control their population yet the existing policies is developing the country, however countries where there is overpopulation especially in some African countries unfortunately hadn't utilize that opportunity due to high level of corruption by their rulers.
Sizeable population doesn't mean higher purchasing power for example India is the most populous country in the world and despite that Japan, with just Eight percent of that population of India is having a higher purchasing power. It depends on a lot of things other than population such as education, skillset and market condition. In some cases having a larger population can be a disadvantage. This can be true if the population is mostly uneducated and dependent on government subsidies and freebies also if the population is divided based on caste and quantity, because the government has given priority to those group, which has a larger quantity and there the priority may not be for economic development