Stellar mass assembly for the simulated and observed samples at z = 0. The upper panel shows the stellar mass fraction over time for each compact subhalo selected from TNG50, color-coded by the stellar mass growth fraction since z = 2. The lower panel shows the stellar mass assembly over time for the MaNGA compact galaxies of Grèbol-Tomàs et al. (2023), color-coded by group: blue represents Groups B and C, which show extended and/or late SFHs; red represents galaxies in their Group A, which exhibit early and rapid formation (and thus negligible accretion after z = 2); dark red indicates extreme cases, where the galaxy is fully assembled early on (with < 10% accreted material). In both panels, the dashed vertical black line indicates z = 2 and the dashed horizontal gray line indicates the stellar mass fraction of 90%.