Morphology of SMGs and field galaxies as a function of wavelength. a) The growth curve half-light radius (R_h) of the SMGs, normalised by the rest-frame 1 μm half-light radius of the respective galaxy as a function of rest-frame wavelength and colour-coded by the galaxy integrated A_V. We show individual measurements for SMGs, and a representative error bar, as well as the running median for the field sample (grey points and line) and massive field sample (black points and line). On the right-hand axis we show the normalised half-light radius scaled by the median rest-frame 1 μm size ($R_{\mathrm{h}}^{1\mathrm{\mu }\mathrm{m}}$ = 3.1 kpc) for the SMGs. b) Sérsic index derived from the galfitm parametric fitting as a function of rest-frame wavelength. We indicate the NIRCam measurements by circles and MIRI measurements by squares. We show a running median, and standard error ($\sigma /\sqrt{n}$) in fixed logarithmic bins of wavelength for the SMGs and field galaxies. In both panels, we plot the parametric fit for the field (solid grey line) and SMGs (solid red line). We find that the SMGs, on average, have a stronger size variation with wavelength, becoming more compact at longer wavelengths quicker than either field sample, whilst exhibiting lower Sérsic index at all wavelengths.