Present-day stellar half-mass radius vs. stellar mass (within 2 stellar half-mass radii), highlighting our adopted samples of Compacts and Normals (both centrals and satellites). The orange, blue, and green circles are the different samples, respectively: Normals, Compacts_MB (Main branch) and Compacts_SB (Secondary branch), while the gray dots are all the subhalos in the stellar mass range. The triangles are bad-flag galaxies. The red solid line is the separation made with a Gaussian mixing model (GMM).