Distribution of non-parametric total sizes, , as measured in the Hubble/ACS F814W imaging of spec-z member galaxies of PCl J1001+0220 compared to that of the coeval (red and blue histograms, respectively). As in previous figures, histograms are normalized to have equal areas. Due to the relationship between size and stellar mass, size measurements are modulated by the estimated stellar mass of each galaxy () where α is derived from a fit to the observed size-stellar mass relation in the combined proto-cluster and coeval field sample (α = 0.18 ± 0.03). The average value for both samples is identical within the uncertainties, though a slight increase (~25%) in the spread of the proto-cluster distribution is observed which we argue in Sect. 4.2.3 that, if real, may be due to increased galaxy–galaxy interactions.