Evolution of two typical compact ETGs. A merger-free galaxy TNG50 475619 is shown on the left and a strongly stripped ETG TNG50 184937 on the right. Top: Evolution of the main progenitor galaxy in the stellar mass versus half-mass radius, colour-coded by the redshift as indicated by the colour bar. The star marks its position at z = 0, the insert panels show the edge-on and face-on surface brightness of the galaxy at z = 0. Bottom: Evolution of the main progenitor galaxy in the mass fraction of the hot inner stellar halo f_∗,halo versus disk f_∗,disk. The insert panels display the stellar orbit distribution p(r, λ_z) of the galaxy at z = 0. In the right panels for TNG50 184937, the red arrow points to the direction of evolution by a major merger, the green arrow indicates the regrowth of a disk, and the magenta arrow indicates tidal stripping.