Cartoon illustrating a physical explanation of the correlation between f_obsc and star-forming compactness. Stars are shown in blue. Diffuse ISM dust is shown with light red and the ‘birth cloud’ dust with higher column density is shown with a darker red colour. Schematically, less and more dust-obscured light (i.e. obscured by the low and high column density dust, respectively) are shown with blue and red arrows, respectively, reaching to the observer on the right. Extended galaxies have a range of dust-star geometries, while compact galaxies contain most of their stellar population in the core with high dust covering fraction, resulting in most of the emitted starlight being obscured by dust.