At fixed stellar mass, the population of passive galaxies (PGs) has increased its mean effective radius by a factor ~5 in the last 10 Gyr, decreasing its mean stellar mass density (S = Mstar/(2πRe^2) by a factor >> 10. Whether this increase in is mainly due to the size-growth of individual galaxies through dry mergers, or to the fact that newly quenched galaxies have a larger size, is still matter of debate. In this context, massive (Mstar~10^11 Mstar) PGs are the most intriguing systems to study, since, in a hierarchical scenario, they are expected to accrete their stellar mass mainly by mergers. Combined studies on the evolution of the number density, on the integrated and spatially resolved stellar population properties (e.g. Age, Metallicity) as derived from optical spectra, as well as on the local environment of massive PGs can provide relevant clues on their mass assembly history. Using the unparelleled statistics of VIPERS survey for massive PGs (~2000 galaxies), and the deep spectroscopy (up to 80h per target) of VANDELS, we have addressed all these topics as a function of S, in the redshift range 0.5 < z <1.5. In this talk I will present our results and conclusions.