Size-size diagram of FWHM in N- and K-bands for group I (blue), group II (red), and unclassified (black) sources. The numbering refers to Table 4. Blue and red dashed lines correspondto an error-bar-weighted linear regression on the group I and group II sources, respectively: y = ax + b with a_I = 5.87 ± 0.14, b_I = −0.50 ± 0.08, a_II = 3.23 ± 0.04, b_II = 1.54 ± 0.09. The dotted lines delimit the uncertainty on the slope parameters. The inset shows a zoom of the overlap region (see text for details).