Redshift zero correlations of CG properties with total group stellar mass. From left to right: i) Mean surface brightness, ${\overline{\mu }}_r$; ii) mean projected radius, ${\overline{R}}_{\mathrm{p}}$; and iii) 1D velocity dispersion, σ_v,1D, for all groups with four or more galaxies in TNG100. The threshold value, ${\overline{\mu }}_r=26.33\mathrm{mag}{\mathrm{arcsec}}^{-2}$ (horizontal dashed red line, left panel), defines the sample of CGs (filled blue circles), while non-CGs (empty red circles) lie above this threshold. CGs are the smallest groups at a given stellar mass, but their velocity dispersions are similar to those of non-CGs. The values of the CG of Fig. 1 are shown with black crosses.