Star-forming size–mass relations in rest-UV (blue) and rest-optical (red) across three redshift bins: 3 ≤ z < 4, 4 ≤ z < 5, and 5 ≤ z < 6 (left to right). The median fit is shown with two shaded regions: the 1σ fit uncertainty and the median intrinsic scatter. The 3 ≤ z < 4 relation is plotted as a black dashed line in all panels. At z < 5, rest-UV and rest-optical relations are within 1σ but at z > 5, star-forming galaxies with log(M_*/M_⊙) < 9 are > 2σ smaller in rest-UV.