Rest-optical size–mass relation across four redshift bins: 3 ≤ z < 4, 4 ≤ z < 5, 5 ≤ z < 6 and 6 ≤ z < 9. The solid black line represents the median fit, with its uncertainty in dark shading and intrinsic scatter in light grey. The grey open circles mark the data points used in the fit. The dotted line indicates the PSF size at the bin’s median redshift; points below this threshold R_EFF were excluded from fitting. In the highest redshift bin, due to limited data, we fixed the slope and fitted only the intercept and intrinsic dispersion (see Section 3.4).