  We study the evolution of the stellar and dark matter components in a galaxy
cluster of $10^{15} \, \rm{M_{\odot}}$ from $z=3$ to the present epoch using
the high-resolution collisionless simulations of Ruszkowski & Springel (2009).
At $z=3$ the dominant progenitor halos were populated with spherical model
galaxies with and without accounting for adiabatic contraction. We apply a
weighting scheme which allows us to change the relative amount of dark and
stellar material assigned to each simulation particle in order to produce
luminous properties which agree better with abundance matching arguments and
observed bulge sizes at $z=3$. This permits the study of the effect of initial
compactness on the evolution of the mass-size relation. We find that for more
compact initial stellar distributions the size of the final Brightest Cluster
Galaxy grows with mass according to $r\propto M^{2}$, whereas for more extended
initial distributions, $r\propto M$. Our results show that collisionless
mergers in a cosmological context can reduce the strength of inner dark matter
cusps with changes in logarithmic slope of 0.3 to 0.5 at fixed radius. Shallow
cusps such as those found recently in several strong lensing clusters thus do
not necessarily conflict with CDM, but may rather reflect on the initial
structure of the progenitor galaxies, which was shaped at high redshift by
their formation process.
