2026ApJ...999..160M

The stellar mass-size relation is a sensitive probe of how environment shapes galaxy structure. We analyse this relation in the local Universe for galaxies in compact groups (CGs), low-mass groups (M_vir ≤ 10¹³M_⊙), and high-mass groups, comparing them to field galaxies using data from the Southern Photometric Local Universe Survey. Galaxies are classified as early-type galaxies (ETGs; n ≥ 2.5, (u - r)₀ ≥ 2.2), late-type galaxies (LTGs; n < 2.5, (u - r)₀ <2.2), transition galaxies (TGs; n < 2.5, (u - r)₀ ≥ 2.2), and other galaxies (OGs; n ≥ 2.5, (u - r)₀ <2.2). We find that ETGs and OGs show no significant environmental dependence: their mass-size slopes and intercepts are statistically consistent across CGs, groups, and the field. LTGs also follow similar relations in the field and in most groups, with only a modest tendency for LTGs in CGs to be smaller at fixed stellar mass. By contrast, TGs display a clear environmental signal: in groups the slope steepens to α ∼ 0.4 (versus α ∼ 0.2 in the field) and their sizes are smaller than in the field, with nonoverlapping 95% posterior intervals. These trends suggest that TGs in denser environments are more structurally evolved, likely owing to enhanced bulge prominence and fading of the outer disk, consistent with the Sérsic-index distributions, which show an excess of TGs with n_r ≳ 1.5 in groups and CGs. Our findings highlight TGs as an environmentally sensitive population, providing insight into the structural transformation of galaxies in group environments.