Where exactly on the map does the size and concentration of the boxes appear to be the highest?

The concentration and size of boxes the same at all locations is not the same size. The deaths due to cholera vary depending upon the location. The relative sizes represents that died at that certain spot.

Explanation:

The black boxes represent relative numbers of death due to cholera. Cholera is referred to as a disease of poverty because of the lack of social development in the areas in which it occurs. Crowded camps are fertile ground for a cholera outbreak. Other humanitarian crises including flooding and earthquakes, civil unrest or war that causes disruption of water and sanitation systems.