The most massive galaxies in clusters are already fully grown at $z \sim
  0.5$
  By constructing scaling relations for galaxies in the massive cluster
MACSJ0717.5 at $z=0.545$ and comparing with those of Coma, we model the
luminosity evolution of the stellar populations and the structural evolution of
the galaxies. We calculate magnitudes, surface brightnesses and effective radii
using HST/ACS images and velocity dispersions using Gemini/GMOS spectra, and
present a catalogue of our measurements for 17 galaxies. We also generate
photometric catalogues for $\sim 3000$ galaxies from the HST imaging. With
these, we construct the colour-magnitude relation, the fundamental plane, the
mass-to-light versus mass relation, the mass-size relation and the
mass-velocity dispersion relation for both clusters. We present a new, coherent
way of modelling these scaling relations simultaneously using a simple physical
model in order to infer the evolution in luminosity, size and velocity
dispersion as a function of redshift, and show that the data can be fully
accounted for with this model. We find that (a) the evolution in size and
velocity dispersion undergone by these galaxies between $z \sim 0.5$ and $z
\sim 0$ is mild, with $R_e(z) \sim (1+z)^{-0.40\pm0.32}$ and $\sigma(z) \sim
(1+z)^{0.09 \pm 0.27}$, and (b) the stellar populations are old, $\sim 10$ Gyr,
with a $\sim 3$ Gyr dispersion in age, and are consistent with evolving purely
passively since $z \sim 0.5$ with $\Delta \log M/L_B = -0.55_{-0.07}^{+0.15}
z$. The implication is that these galaxies formed their stars early and
subsequently grew dissipationlessly so as to have their mass already in place
by $z \sim 0.5$, and suggests a dominant role for dry mergers, which may have
accelerated the growth in these high-density cluster environments.
