Comparison between the isophotal area (region enclosed by an isophote of 10^-18 erg s^-1 cm^-2) and the circular area obtained from the r_eff of the individual star-forming regions in LIRGs (circles) and ULIRGs (diamonds). The thick black line represents the ratio between both quantities for a region with a uniform flux distribution, whereas the dashed line represents the same ratio for a region with a symmetric 2D Gaussian light profile and a total flux of 10^-13 erg s^-1 cm^-2.