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When investigating the sizes of galaxies it is standard practice to use thehalf-light radius, $r_{50}$. Here we explore the effects of the size definitionon the distribution of galaxies in the size -- stellar mass plane.Specifically, we consider $r_{20}$ and $r_{80}$, the radii that contain 20% and80% of a galaxy's total luminosity, as determined from a Sersic profile fit,for galaxies in the 3D-HST/CANDELS and COSMOS-DASH surveys. These radii arecalculated from size catalogs based on a simple calculation assuming a Sersicprofile. We find that the size-mass distributions for $r_{20}$ and $r_{80}$ aremarkedly different from each other and also from the canonical $r_{50}$distribution. The most striking difference is in the relative sizes of starforming and quiescent galaxies at fixed stellar mass. Whereas quiescentgalaxies are smaller than star forming galaxies in $r_{50}$, this differencenearly vanishes for $r_{80}$. By contrast, the distance between the twopopulations increases for $r_{20}$. Considering all galaxies in a given stellarmass and redshift bin we detect a significant bimodality in the distribution of$r_{20}$, with one peak corresponding to star forming galaxies and the other toquiescent galaxies. We suggest that different measures of the size are tracingdifferent physical processes within galaxies; $r_{20}$ is closely related toprocesses controlling the star formation rate of galaxies and $r_{80}$ may besensitive to accretion processes and the relation of galaxies with their halos.

A New View of the Size-Mass Distribution of Galaxies: Using $r_{20}$ and $r_{80}$ instead of $r_{50}$