Color gradients are responsible for most of the evolution in the mass-size relation

Galaxy sizes are a key physical observable, and their growth over cosmic time provides clues to how galaxies evolve. However, the light profiles used to measure galaxy sizes do not directly trace the underlying stellar mass profiles: radial mass-to-light ratio gradients cause half-mass and half-light radii to differ. We use multi-band imaging to measure the half-mass radii of ~7,000 galaxies at 1.0<z<2.5 in the CANDELS fields. We find that half-mass radii are generally smaller than half-light radii. Furthermore, the strength of color gradients evolves with redshift. Between z~2.5 and z~1, half-mass radii grow by only ~1/3 the amount that half-light radii do. Color gradients are thus responsible for much of the previously-discovered size growth of quiescent galaxies. Finally, I will discuss recent work that investigates how the half-mass radii of galaxies vary as a function of the galaxy's rest-frame spectral energy distribution shape.