Mean surface brightness within 300 kpc vs. core radius, color-coded by redshift (useful for the comparison with Fig. 2 and for the discussion) for clusters in eFEDS. Clusters that are also detected in the ACT survey are marked with a square. About 5% of the eFEDS clusters have a mean surface brightness below 43.35 erg s⁻¹ Mpc⁻² vs. about one quarter in the XUCS sample. The ACT survey only detects the clusters with a higher surface brightness. The vertical dotted line marks the minimum core radius we considered to compute the fraction of clusters with a low surface brightness. As for the XXL-100 sample, correcting luminosities for evolution would not significantly alter the results.