Stellar half-mass radius (r_50, *) as function of stellar mass (M_*) for our sample of central TNG50 galaxies, extended to log(M_*/M_⊙) = 7 (virial masses down to log(M₂₀₀/M_⊙)∼10). Each galaxy is represented with a circle coloured according to the fraction of stellar mass enclosed within 1 kpc of its centre f_{*, < 1} = M_{*, < 1}/M_*. Coloured squares highlight galaxies A, B, C, and D from Figure 1. We note that the stellar-mass-size relation seems to ‘converge’ to ∼1 kpc in the dwarf regime, with a secondary population of unusually compact galaxies that reach a very small r_50, * at log(M_*/M_⊙)∼9. These galaxies reach sizes comparable to the gravitational-force softening length for stars and dark-matter particles (0.29 kpc at redshift z = 0) and correspond to systems where the central clump dominates not only the galaxy stellar mass budget, but also the total mass budget within 1 kpc. See text for further discussion.