ETG evolution in stellar mass, effective radius, and effective velocity dispersion, due to major and minor mergers. Upper panel: Effective radius, R_e, as a function of stellar mass, M_⋆. The lines indicate the median observed R_e-M_⋆ relations of ETGs at different redshifts (van der Wel et al. 2014). Lower panel: Effective stellar velocity dispersion, σ_e, as a function of stellar mass, M_⋆. The lines indicate the median observed σ_e-M_⋆ relations of ETGs at different redshifts (Cannarozzo et al. 2020). In each panel the black arrow indicates the predicted evolution of a representative model ETG from z = 3 (tail of the arrow) to z = 0 (head of the arrow), under the effect of major (ξ > 1/4) and minor (1/10 < ξ < 1/4) dissipationless mergers occurring at the rates observationally estimated by Conselice et al. (2022). The grey area represents the uncertainty in the position of the head of the arrow. The blue and red error bars indicate the intrinsic scatter of the scaling relations at z = 0 and z = 3, respectively.