  Kuiper Belt objects with absolute magnitude less than 3 (radius $\gtrsim$500
km), the dwarf planets, have a range of different ice/rock ratios, and are more
rock-rich than their smaller counterparts. Many of these objects have moons,
which suggests that collisions may have played a role in modifying their
compositions. We show that the dwarf planets fall into two categories when
analysed by their mean densities and satellite-to-primary size ratio. Systems
with large moons, such as Pluto/Charon and Orcus/Vanth, can form in
low-velocity grazing collisions in which both bodies retain their compositions.
We propose that these systems retain a primordial composition, with a density
of about 1.8 g/cm$^3$. Triton, thought to be a captured KBO, could have lost
enough ice during its early orbital evolution to explain its rock-enrichment
relative to the primordial material. Systems with small moons, Eris, Haumea,
and Quaoar, formed from a different type of collision in which icy material,
perhaps a few tens of percent of the total colliding mass, is lost. The
fragments would not remain in physical or dynamical proximity to the parent
body. The ice loss process has not yet been demonstrated numerically, which
could be due to the paucity of KBO origin simulations, or missing physical
processes in the impact models. If our hypothesis is correct, we predict that
large KBOs with small moons should be denser than the primordial material, and
that the mean density of Orcus should be close to the primordial value.
