The whole sub-saharan Africa was once colonised by European countries apart from one country called Liberia. Liberia was not colonised cos of American protection as slaves from america were released there. One major rule followed by colonial powers was back then was divide and conquer. They divided countries in a way that tribes were divided among countries at first. After the WW2, the colonial powers even have more chances to practice this rule as they got more area as Germans countries were divided among them. The effect of this can be seen today on the african map with every country is divided along rivers while dividing some tribes and communities while forcing some communities to live together under one flag. So, TLDR, country big enough to be the size of China in Africa wont be good for colonial leadere, hence countries were drawn in smaller sizes.