Baryonic mass–size relations considering all baryons (left panel) and the baryonic disks only (excluding stellar bulges, right panel). Galaxies are colour-coded by the gas fraction f_gas. Crosses represent S0-to-Sc galaxies (Hubble type T ≤ 5), while circles represent Sd-to-dI galaxies (T > 5). The red and the blue solid lines show the best-fit relations to the HSD and LSD sequences, respectively; the red and blue shaded areas indicate the best-fit intrinsic scatters. Dashed lines correspond to constant surface densities for multiples of Σ_c = 125 M_⊙ pc⁻² (see Sect. 3 for details).