When the Subthreshold Boxes button in the Main Window sub-menu of the Attributes Menu or the sub-window Attributes menu is active, subcells with a displayed size smaller than a threshold will be shown in unexpanded form, as an unfilled box. Otherwise, the cell will not be shown at all.
The threshold pixel size can be adjusted with the Subcell visibility threshold (pixels) entry area in the Main Window Attributes panel, or equivalently by setting the CellThreshold variable.