Left: SFR – M _∗  correlation at z ~ 0. Galaxies classified as compact are marked with large filled red dots. Solid line: fit to the main sequence SFR–M _∗  relation: SFR  ∝  M _⋆ /[4  ×  10⁹ M_⊙] . Dotted lines: 16th and 84th percentiles of the distribution around the sliding median (0.26 dex). Right: relation of the sSFR and IR surface brightness of galaxies for which a radio size was estimated. The vertical dashed line illustrates the threshold above which galaxies have been classified as compact. The solid and dashed red lines are a fit to the sliding median of the relation (Eq. (11)) and its 68% dispersion.