Comparison between the 3D spherical half-mass radius, r_{1/2, mass, 3D}, and the projected 2D effective radius, R_e, for a range of Sérsic indices n and intrinsic axis ratios q₀ (left, colored by q₀; right, colored by n). For oblate cases, R_e is the projected major axis, while for prolate cases R_e is the projected minor axis. For all cases, r_{1/2, mass, 3D} > R_e. However, as q₀ decreases (i.e., flatter Sérsic distributions), the 3D half-mass radius approaches the value of R_e. Overall, the systematic difference between r_{1/2, mass, 3D} and R_e highlights that while half of the model mass is enclosed within a projected 2D ellipse of major axis R_e (e.g., an infinite ellipsoidal cylinder), less than half the total mass is enclosed within a sphere of radius R_e (ignoring any M/L gradients or optically thick regions, which would change R_e, light/R_e, mass).