Mass-radius relations (MRRs) of our model galaxies labeled by their present-day total baryonic mass M_B(T_G) equal to 10⁶, 10⁸, 10¹⁰, and 10¹² M_⊙ (from left to right). Each line made of filled black squares represents the whole evolutionary history of M_s and R_e, both increasing with time (present-day at the top). These models are compared both with observational and theoretical data from different sources: (i) the observational data of Burstein et al. (1997) from GCs to GCGs (light blue dots) and the ETGs by Bernardi et al. (2010, red dots); (ii) the Illustris-1 galaxies (light green dots); (iii) the low initial density models (blue squares and their best fit) and the high initial density models (red squares and their best fit) by Chiosi & Carraro (2002; iv) the early hierarchical models by Merlin et al. (2012, black squares and their best fit); (v) the Fan et al. (2010) MRRs for different values of the formation redshift (from top to bottom) z_f = 0, 1, 5, 10, and 20; and (vi) the MRR by Chiosi et al. (2020, dark yellow line). See the text for more details.