Newly-quenched galaxies as the cause for the apparent evolution in average size of the population

We study the number densities of quenched early-type galaxies (Q-ETGs) of a given size in a homogeneous COSMOS sample over the interval 1 < z < 0.2. At 10^10.5<M<10^11 Msun we see no change in the number density of compact Q-ETGs, while at >10^11 Msun we find a decrease by 30-40%. In both mass bins, the increase of the median sizes of Q-ETGs with time is primarily caused by the addition of larger Q-ETGs. At all masses, compact Q-ETGs become redder towards later epochs, with a (U-V) color difference which is consistent with a passive evolution of their stellar populations. At all epochs, the larger Q-ETGs have colors that are systematically bluer than those of the more compact Q-ETGs, suggesting that the former are younger than the latter. The sizes and number of the star-forming galaxies over the same redshift interval also support the idea that new Q-ETGs are responsible for the growth in the median size of the population at a given mass. The sizes of the new members of the population appear to scale as roughly (1+z)^-1. We conclude that the dominant cause of the apparent size evolution of the population of Q-ETGs is the addition at later epochs of larger galaxies. [abridged]