City population size is a crucial measure to understand urban life. Here, we analyse the impact of city population size on the emergence of nearby cities. Based on a road network, we construct the region of influence of African mainland cities. We then use urban scaling models to describe the relationship between city population size and the number of cities or the total population within each region of influence. For small values of the radius of the constructed regions of influence, we observe a sublinear scaling behaviour, but as the radius is increased, this behaviour becomes closer to linear.