Distributions of BCG properties, from left to right: ^0.1r-band absolute magnitude; r-band surface brightness in mag arcsec^-1; Petrosian half-light radius in h^-1kpc; concentration parameter; ^0.1(u − r) colour; stellar mass; and the probability of being elliptical (p_E) or spiral (p_S). In the top and middle rows, thick black line corresponds to the sample of compact groups, blue line to low mass loose groups, and red line to high mass loose groups. The bottom row compares the equal luminosity sub-samples of compact (thick black) and loose groups (green). All distributions have been normalised to have the same area. Below each panel, we show as shaded histograms the residuals between the distributions. We quote in each panel the probability of the null hypothesis that both data sets are drawn from the same distribution according to the Kolmogorov-Smirnov test.