Slums are litmus tests for innate cultural strengths and weaknesses. Those peoples whose cultures can harbor extensive slum life without decomposing will be, relatively speaking, the future’s winners. Those whose cultures cannot will be the future’s victims. Robert D. Kaplan, The Coming Anarchy, 1994.
The legend’s numbers are by the millions (4.0=4 million) of inhabitants*. Multiple circles indicate multiple mega slums. For a full breakdown of the raw data (including another interesting map) see this wikimedia
page or the maps author page (Walké). Nod to casuist for this.
*The map illustrates data from “mega slums” with populations greater than 500,000.