  We revisit collisionless major and minor mergers of spheroidal galaxies in
the context of the size evolution of elliptical galaxies. The simulations are
performed as a series of mergers with mass-ratios of 1:1 and 1:10 for models
representing pure bulges as well as bulges embedded in dark matter halos. For
major and minor mergers, respectively, we identify and analyze two different
processes, violent relaxation and stripping, leading to size evolution and a
change of the dark matter fraction within the observable effective radius.
Violent relaxation - which is the dominant mixing process for major mergers but
less important for minor mergers - scatters relatively more dark matter
particles than bulge particles to small. Stripping in minor mergers assembles
stellar satellite particles at large radii in halo dominated regions of the
massive host. This strongly increases the size of the bulge into regions with
higher dark matter fractions leaving the inner host structure almost unchanged.
A factor of two mass increase by minor mergers increases the dark matter
fraction by 20 per cent. We present analytic corrections to simple
one-component virial estimates for the evolution of the gravitational radii. If
such a two-component system grows by minor mergers alone its size growth,
$r_{\mathrm{e}} \propto M^\alpha$, reaches values of $\alpha \approx 2.4$,
significantly exceeding the simple theoretical limit of $\alpha = 2$. For major
mergers the sizes grow with $\alpha \lesssim 1$. Our results indicate that
minor mergers of galaxies embedded in massive dark matter halos provide a
potential mechanism for explaining the rapid size growth and the build-up of
massive elliptical systems predicting significant dark matter fractions and
radially biased velocity dispersions at large radii (abbreviated)
