Similar to Fig. 2 but showing the ratio of effective radii (left) and total fluxes (right) obtained from fitting Sérsic models to simulated galaxies created using Spergel profiles (crosses). The data are plotted as a function of their Spergel index. The solid curves denote the best-fit empirical relations as expressed by Eq. (4). The bottom panels show the normalized residuals to the best fit, which are largely within 10%.