A disadvantage of proportional symbolization is that the symbol size can appear
variable depending on the surrounding symbols. This is best shown via the
Ebbinghaus illusion (also known as Titchener circles). As you can see in Figure 9.15
"Ebbinghaus Illusion", the central circles are both the same size but appear
different due to the visual influence of the surrounding circles. If you are creating a
graphic with many different symbols, this illusion can wreak havoc on the
interpretability of your map.