  Using a high resolution hydrodynamical cosmological simulation of the
formation of a massive spheroidal galaxy we show that elliptical galaxies can
be very compact and massive at high redshift in agreement with recent
observations. Accretion of stripped in-falling stellar material increases the
size of the system with time and the central concentration is reduced by
dynamical friction of the surviving stellar cores. In a specific case of a
spheroidal galaxy with a final stellar mass of $1.5 \times 10^{11} M_{\odot}$
we find that the effective radius $r_e$ increases from $0.7 \pm 0.2 \rm kpc$ at
z = 3 to $r_e = 2.4 \pm 0.4 \rm kpc$ at z = 0 with a concomitant decrease in
the effective density of an order of magnitude and a decrease of the central
velocity dispersion by approximately 20% over this time interval. A simple
argument based on the virial theorem shows that during the accretion of weakly
bound material (minor mergers) the radius can increase as the square of the
mass in contrast to the usual linear rate of increase for major mergers. By
undergoing minor mergers compact high redshift spheroids can evolve into
present-day systems with sizes and concentrations similar to observed local
ellipticals. This indicates that minor mergers may be the main driver for the
late evolution of sizes and densities of early-type galaxies.
