(Sub)mm angular major axis FWHM sizes of our SMGs (at 890 μm from Younger et al. 2007, 2008, 2009, at 870 μm from Younger et al. 2010, or at 1.3 mm from Miettinen et al. 2015) plotted against their 3 GHz major axis FWHM sizes. For AzTEC1, 3, 4, 5, 8, and 21a the (sub)mm emission has been (marginally) resolved. For AzTEC1, 4, and 8 we plot the Gaussian-fit size from Younger et al. (2008, 2010), while the size of AzTEC3 is that determined from ALMA observations at 1 mm by Riechers et al. (2014). For AzTEC5, the submm size was derived from the 994 μm ALMA image (Miettinen et al., in prep.) using JMFIT (see text for details). Similarly, the plotted 1.3 mm size of AzTEC21a was derived by fitting the source using JMFIT for a better comparison with the present 3 GHz size. The AzTEC27 data point is not shown due to its non-Gaussian shape at 1.3 mm (see text for details). Among AzTEC2–15, the upper 890 μm size limits – marked with arrows pointing down – are (Younger et al. 2007, 2009), while the upper limits to the 1.3 mm sizes of AzTEC17a, 19a, and 24b are set to one-half the synthesised beam major axis size at FWHM (Miettinen et al. 2015; Table 2 therein). The red dashed line indicates where the radio and (sub)mm sizes are equal.