Size of the dynamically hot spheroid as an indicator of accreted stellar mass fraction f_∗,accreted. Left: Spheroid mass M_∗Sph versus spheroid size R_Sph. TNG50 galaxies are shown as dots coloured by their accreted fraction f_∗,accreted. Black ‘x’ symbols mark the 15 compact ETGs from Yıldırım et al. (2017) as listed in Table 1. Right: Spheroid size R_Sph versus accreted fraction f_∗,accreted for TNG50 galaxies with M_∗Sph > 10¹⁰ M_⊙.