  We present results on the structural properties of massive passive galaxies
in three clusters at $1.39<z<1.61$ from the KMOS Cluster Survey. We measure
light-weighted and mass-weighted sizes from optical and near-infrared Hubble
Space Telescope imaging and spatially resolved stellar mass maps. The
rest-frame $R$-band sizes of these galaxies are a factor of $\sim2-3$ smaller
than their local counterparts. The slopes of the relation between the stellar
mass and the light-weighted size are consistent with recent studies in clusters
and the field. Their mass-weighted sizes are smaller than the rest frame
$R$-band sizes, with an average mass-weighted to light-weighted size ratio that
varies between $\sim0.45$ and $0.8$ among the clusters. We find that the median
light-weighted size of the passive galaxies in the two more evolved clusters is
$\sim24\%$ larger than for field galaxies, independent of the use of
circularized effective radii or semi-major axes. These two clusters also show a
smaller size ratio than the less evolved cluster, which we investigate using
color gradients to probe the underlying $M_{*}/L_{H_{160}}$ gradients. The
median color gradients are $\nabla{z-H} \sim-0.4$ mag dex$^{-1}$, twice the
local value. Using stellar populations models, these gradients are best
reproduced by a combination of age and metallicity gradients. Our results favor
the minor merger scenario as the dominant process responsible for the observed
galaxy properties and the environmental differences at this redshift. The
environmental differences support that clusters experience accelerated
structural evolution compared to the field, likely via an epoch of enhanced
minor merger activity during cluster assembly.
