Stellar half mass radius (r_50, *) as a function of stellar mass (M_*) for all isolated central galaxies in TNG50-1 (left panel, similar to Figure 11) and in TNG100-1 (right panel). All galaxies shown have more than 100 stellar particles and more than 100 dark matter particles. Red dashed lines indicate the force softening length for dark matter and stellar particles for each simulation. The "tongue" is also present in TNG100, but at slightly higher masses and with larger size, suggesting that this feature is numerical in origin.