Occasionally not only the used disk space is interesting, but also the number of files (e.g. thousands of files within one directory causing poor performance). Therefore I'd suggest to display the number of files in addition to the occupied space (maybe in a line below). And, allow to switch the view from "occupied space per directory" to "files per directory".
This should be easy to implement, because the number of files can be computed while computing the space occupied per directory.