‘Dynamical mass-to-light ratio’, , as a function of galaxy effective radius, expressed in units of the ‘critical’ radius introduced in Fig. 4, for EAGLE (left-hand panel) and TNG (right-hand panel). Grey symbols represent all simulated galaxies, while those highlighted in colour correspond to galaxies with stellar masses M_* = 10^{10.5 ± 0.05} M_⊙ (i.e. those in the second grey band, from bottom to top, in Fig. 2). Galaxies in that mass bin are coloured according to κ_rot, going from slow- (red) to fast-rotator galaxies (blue). It is clear that the dynamical mass-to-light ratio, which measures a galaxy’s dark matter content, is an increasing function of R_e/R_crit. This suggests that galaxy size is a primary cause of the tilt of the FP.