  We present a simple, empirically motivated model that simultaneously predicts
the evolution of the mean size and the comoving mass density of massive
early-type galaxies from z=2 to the present. First we demonstrate that some
size evolution of the population can be expected simply due to the continuous
emergence of early-type galaxies. SDSS data reveal that in the present-day
universe more compact early-type galaxies with a given dynamical mass have
older stellar populations. In contrast, at a given stellar velocity dispersion,
SDSS data show that there is no relation between size and age, which implies
that the velocity dispersion can be used to estimate the epoch at which
galaxies stopped forming stars, turning into early-type galaxies. Applying such
a 'formation' criterion to a large sample of nearby early-type galaxies, we
predict the redshift evolution in the size distribution and the comoving mass
density. The resulting evolution in the mean size is roughly half of the
observed evolution. Then we include a prescription for the merger histories of
galaxies between the 'formation' redshift and the present, based on
cosmological simulations of the assembly of dark matter halos. Such mergers
after the transformation into an early-type galaxy are presumably
dissipationless ('dry'), where the increase in size is expected to be
approximately proportional to the increase in mass. This model successfully
reproduces the observed evolution since z~2 in the mean size and in the
comoving mass density of massive early-type galaxies. We conclude that the
recently measured, substantial size evolution of early-type galaxies can be
explained by the combined effect of the continuous emergence of galaxies as
early types and their subsequent growth through dry merging.
