Issues bordering around population studies have caught the attention of scholars in different fields. Inter alia, development economists and demographers have written plethora of literatures on it considering the multidimensional impacts of changing populations on the very existence of humans. Most developing countries are considered to have alarming population growth rates. Matter-of-factly, countries with seriously large population like China have had to adopt some population control measures. In this study, special reference is made to the population theory of Rev. Thomas Malthus and its relevance to the economies of these two countries. This study also re-examines the arguments of both the pessimistic and the optimistic population schools, and analysing relevant demographic and economic statistics of Nigeria and Ethiopia – the most populous and second most populous countries respectively in the African continent, draws a conclusion consistent with the view point of a world development report that rapid population growth, above all, is a developmental problem.