Galaxy Sizes Since $z=2$ from the Perspective of Stellar Mass
  Distribution within Galaxies
  How stellar mass assembles within galaxies is still an open question. We
present measurements of the stellar mass distribution on kpc-scale for
$\sim5500$ galaxies with stellar masses above
$\log(M_{\ast}/M_{\odot})\geqslant9.8$ up to the redshift $2.0$. We create
stellar mass maps from Hubble Space Telescope observations by means of the
pixel-by-pixel SED fitting method. These maps are used to derive radii
encompassing $20\%$, $50\%$, and $80\%$ ($r_{20}$, $r_{50}$ and $r_{80}$) of
the total stellar mass from the best-fit S\'ersic models. The reliability and
limitations of the structural parameter measurements are checked extensively
using a large sample ($\sim3000$) of simulated galaxies. The size-mass
relations and redshift evolution of $r_{20}$, $r_{50}$ and $r_{80}$ are
explored for star-forming and quiescent galaxies. At fixed mass, the
star-forming galaxies do not show significant changes in their $r_{20}$,
$r_{50}$ and $r_{80}$ sizes, indicating self-similar growth. Only above the
pivot stellar mass of $\log(M_{\ast}/M_{\odot})\simeq10.5$, $r_{80}$ evolves as
$r_{80}\propto(1+z)^{-0.85\pm0.20}$, indicating that mass builds up in the
outskirts of these systems (inside-out growth). The S\'ersic values also
increase for the massive star-forming galaxies towards late cosmic time.
Massive quiescent galaxies show stronger size evolution at all radii, in
particular the $r_{20}$ sizes. For these massive galaxies, S\'ersic values
remain almost constant since at least $z\sim1.3$, indicating that the strong
size evolution is related to the changes in the outer parts of these galaxies.
We make all the structural parameters publicly available.
