Left panel: enlargement of the log(⟨I⟩_e)−​log(R_e) plane with real and simulated galaxies. Black filled circles indicate our BCGs, red filled circles our II-BCGs, green filled symbols our random sample of normal ETGs, gray filled dots the WINGS ETGs of D’Onofrio et al. (2017), empty stars the faint ETGs of Bettoni et al. (2016), and filled circles our clusters. The small red dots are used for the whole set of Illustris galaxies at z = 0. In this case the effective mass radius is assumed to be equal to the effective radius. The dotted line is that expected for systems of luminosity M_V = −21.5. Right panel: log(⟨I⟩_e)−​log(R_e) plane for cluster (black dots) and non-cluster (red dots) ETGs. Shown is the subsample of galaxies with available masses from Fritz et al. (2007). In both panels the dashed lines are the trends for virialized systems with slope −1 and a zero point of a possible ZoE.