Scalings of FWHM size at 70 μm (left) and of corresponding rest frame FIR surface brightness (right) with distance from the Renzini & Peng (2015) star-forming main-sequence. The figure includes 86 galaxies with published stellar masses. The red line is a power law fit. Vertical dotted lines indicate a range of ±0.6 dex around the main-sequence, as often used to separate main-sequence galaxies from bursting or passive objects.