  We investigate the origin of the evolution of the population-averaged size of
quenched galaxies (QGs) through a spectroscopic analysis of their stellar ages.
The two most favoured scenarios for this evolution are either the size growth
of individual galaxies through a sequence of dry minor merger events, or the
addition of larger, newly quenched galaxies to the pre-existing population
(i.e., a progenitor bias effect). We use the 20k zCOSMOS-bright spectroscopic
survey to select bona fide quiescent galaxies at 0.2<z<0.8. We stack their
spectra in bins of redshift, stellar mass and size to compute stellar
population parameters in these bins through fits to the rest-frame optical
spectra and through Lick spectral indices. We confirm a change of behaviour in
the size-age relation below and above the ~10^11 MSun stellar mass scale: In
our 10.5 < log M*/MSun < 11 mass bin, over the entire redshift window, the
stellar populations of the largest galaxies are systematically younger than
those of the smaller counterparts, pointing at progenitor bias as the main
driver of the observed average size evolution at sub-10^11 MSun masses. In
contrast, at higher masses, there is no clear trend in age as a function of
galaxy size, supporting a substantial role of dry mergers in increasing the
sizes of these most massive QGs with cosmic time. Within the errors, the
[alpha/Fe] abundance ratios of QGs are (i) above-solar over the entire redshift
range of our analysis, hinting at universally short timescales for the buildup
of the stellar populations of QGs, and (ii) similar at all masses and sizes,
suggesting similar (short) timescales for the whole QG population and
strengthening the role of mergers in the buildup of the most massive QGs in the
Universe.
