Mean effective surface brightness versus the effective radius in the r-band. Galaxies with well constrained optical parameters (see Sect. 3.3 for more details) are in color, with UDGs as cyan stars and standard dwarf galaxies as purple diamonds. The rest of the sample with less reliable parameters are shown as white markers, with UDGs as stars and standard dwarfs as diamonds. The H I selected UDG sample from Leisman et al. (2017) is plotted as gray circles, and the sample from M20 as red hexagons. The dash dotted line corresponds to the threshold of surface brightness used for the classification of UDGs.