Stellar mass M_* versus size R_e Compact ETGs from observations and the Illustris TNG50 simulation. The dots denote TNG50 galaxies coloured by their specific star formation rate log(sSFR) at z = 0. Grey ‘x’ symbols denote the 15 compact ETGs from observations in Yıldırım et al. (2017). Black dashed lines indicate the mass-size relations for galaxies observed at z = 0.25, z = 1.25, and z = 2.25 (van der Wel et al. 2014). Compact ETGs from TNG50 below the red line are selected with $M_{\ast }/R_{\mathrm{e}}^{1.5}\equiv {\mathrm{\Sigma }}_{1.5}>{10}^{10.2}{M}_{\odot }{\mathrm{kpc}}^{-1.5}$ (Barro et al. 2013) and with log(sSFR) <  − 11.