From the D3 documentation:
Enclosure diagrams use containment (nesting) to represent the hierarchy. The size of each leaf node’s circle reveals a quantitative dimension of each data point. The enclosing circles show the approximate cumulative size of each subtree, but note that because of wasted space there is some distortion between levels; only the leaf nodes can be compared accurately. Although circle packing does not use space as efficiently as a treemap, the “wasted” space more prominently reveals the hierarchy.
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