the same underlying distribution in $\Sigma_{1.5}$ with a significance $>99.99$%, with the same significance obtained with either value of $\alpha$.

Given the strong evolution of the PSB mass function (Wild et al. 2016), a concern is that massive PSBs are more common at higher redshifts, which may bias the size–mass comparison when measured over a wide redshift range. In Figure \[fig:size-mass-z12\] we therefore display the size–mass relation in two narrower redshift bins, $1.0<z<1.5$ and $1.5<z<2.0$. With two independent samples, the results confirm that post-starburst galaxies, on average, show smaller half-light radii than the passive population at high mass (M$_{\ast}> 10^{10.5} ~$M$_{\sun}$). An additional test was performed using a weighted mean, with a redshift-dependent weight determined for each PSB using the ratio of the $n(z)$ distributions for passive galaxies and PSBs. The resulting size–mass relations were barely changed, with only a slight reduction in the significance of the differences reported above.

A natural interpretation of our findings is that quiescent galaxies are most compact when they are newly-quenched, but then grow with cosmic time. Given the short-lived nature of the PSB phase, we expect the majority of passive galaxies to have gone through a similar stage in their past (Wild et 