Evolution of q_IR as a function of SFR surface density (Σ_SFR = SFR_IR + UV/2), colour coded by redshift (left panel) and by M_⋆ (right panel). The SFR_IR + UV estimates are taken from Sect. 3.3, while the effective radius R_e is measured from stacked 3 GHz images via 2D Gaussian fitting. This plot shows a significant anti-correlation similar in slope to that observed with M_⋆ (Sect. 4.3), marked by the black dashed line (q_IR ∝ ( − 0.13 ± 0.02) × logΣ_SFR). For comparison, the best-fit trend with rest-frame optical (5000 Å) sizes estimated from van der Wel et al. (2014) scaling relation is shown (grey dotted line).