Comparison between observed and model-predicted sizes and colours. Left: observed half-light radius as a function of the average R_e of galaxies with the same redshift and luminosity, as predicted by Eq. (6). Right: observed rest-frame u − g colour as a function of the average colour of galaxies with the same redshift, luminosity, and size, as predicted by Eq. (8). Uncertainties on the model parameters are neglected, as they are very small with respect to the observational scatter. The dashed line marks the 1 : 1 relation.