  Using deep Hubble Frontier Fields imaging and slitless spectroscopy from the
Grism Lens-Amplified Survey from Space, we analyze 2200 cluster and 1748 field
galaxies at $0.2\leq z\leq0.7$ to determine the impact of environment on galaxy
size and structure at $\log M_*/M_\odot>7.8$, an unprecedented limit at these
redshifts. Based on simple assumptions-$r_e=f(M_*)$-we find no significant
differences in half-light radii ($r_e$) between equal-mass cluster or field
systems. More complex analyses-$r_e=f(M_*,U-V,n,z,\Sigma$)-reveal local density
$(\Sigma$) to induce only a $7\% \pm 3\%$ ($95\%$ confidence) reduction in
$r_e$ beyond what can be accounted for by $U-V$ color, Sersic index ($n$), and
redshift ($z$) effects.Almost any size difference between galaxies in high- and
low-density regions is thus attributable to their different distributions in
properties other than environment. Indeed, we find a clear color-$r_e$
correlation in low-mass passive cluster galaxies ($\log M_*/M_\odot<9.8$) such
that bluer systems have larger radii, with the bluest having sizes consistent
with equal-mass star-forming galaxies. We take this as evidence that
large-$r_e$ low-mass passive cluster galaxies are recently acquired systems
that have been environmentally quenched without significant structural
transformation (e.g., by ram pressure stripping or starvation).Conversely,
$\sim20\%$ of small-$r_e$ low-mass passive cluster galaxies appear to have been
in place since $z\sim3$. Given the consistency of the small-$r_e$ galaxies'
stellar surface densities (and even colors) with those of systems more than ten
times as massive, our findings suggest that clusters mark places where galaxy
evolution is accelerated for an ancient base population spanning most masses,
with late-time additions quenched by environment-specific mechanisms are mainly
restricted to the lowest masses.
