  The size-luminosity relation of early-type Brightest Cluster Galaxies (BCGs),
R_e ~ L^0.88, is steeper than that for the bulk of the early-type galaxy
population, for which R_e ~ L^0.68. In addition, although BCGs are hardly
offset from the Fundamental Plane defined by the bulk of the early-type
population, they show considerably smaller scatter. The larger than expected
sizes of BCGs, and the increased homogeneity, are qualitatively consistent with
models which seek to explain the colors of the most massive galaxies by
invoking dry dissipationless mergers, since dissipation tends to reduce the
sizes of galaxies, and wet mergers which result in star formation would tend to
increase the scatter in luminosity at fixed size and velocity dispersion.
Furthermore, BCGs define the same g-r color-magnitude relation as the bulk of
the early-type population. If BCGs formed from dry mergers, then BCG
progenitors must have been red for their magnitudes, suggesting that they
hosted older stellar populations than typical for their luminosities. Our
findings have two other consequences. First, the R_e-L relation of the
early-type galaxy population as a whole (i.e., normal plus BCG) exhibits some
curvature. Some of this curvature must be a consequence of the fact that an
increasing fraction of the most luminous galaxies are BCGs. The second
consequence is suggested by the fact that, despite following a steeper
size-luminosity relation, BCGs tend to define a tight relation between
dynamical mass R_e sigma^2/G and luminosity. As consequence, we find that BCGs
define a shallower sigma-L relation than the bulk of the early-type galaxy
population.
