Relation between the specific central mass density ${\mathrm{\Sigma }}_{1,\mathrm{kpc}}^{\mathrm{spec}}$ and the global properties of galaxies. Left panel: ${\mathrm{\Sigma }}_{1,\mathrm{kpc}}^{\mathrm{spec}}$ as a function of stellar mass. Right panel: ${\mathrm{\Sigma }}_{1,\mathrm{kpc}}^{\mathrm{spec}}$ versus R₁. In both panels, simulated galaxies from IllustrisTNG50 are colour-coded according to their bulge prominence, with BD galaxies (orange diamonds) occupying the upper regions of the relations, while BL systems (green circles) lie systematically below. BI galaxies (blue crosses) span a wide range in both ${\mathrm{\Sigma }}_{1,\mathrm{kpc}}^{\mathrm{spec}}$ and size, bridging the two extremes. Observational data from the BEARD sample are shown as light-green squares, consistently occupying the region where BL galaxies are located.