Proportional circle maps

A Proportional circle map measures variables at different locations on a map by placing circles of different sizes to represent the comparative size of the variable in the locations.  The map here uses the shapes to measure the number of Walmarts within the respective states.  When compared to my cartograms map you can see there are conflicting numbers in the numbers of Walmarts.  I doubt one year of difference could explain away the variance. 

http://personal.frostburg.edu/sbriggs0/maps.htm