Mean surface brightness within 300 kpc vs. core radius for the XXL-100 sample, color-coded by redshift (useful for a comparison with Fig. 3 and for the discussion). Only about 1% of the clusters has r_c > 100 kpc and a mean surface brightness below 43.35 erg s⁻¹ Mpc⁻² (blue line) vs. one quarter in the XUCS sample. The vertical dotted line marks the minimum core radius we considered to compute the fraction of clusters with a low surface brightness. Correcting luminosities for evolution would only alter the points in the top part of the figure, which is of no interest here.