Mass-to-size plot for the 76 methyl formate sources extracted from the moment 0 maps (Figs. 3–6). The different symbols represent the different evolutionary stages of the protoclusters: young, intermediate and evolved, as indicated on top of the figure, while the stars shows the methyl formate sources that are not associated with any compact dust continuum core. The error bars show the range of masses obtained considering the uncertainties on T_d (300 ± 100K for the most extreme objects and 100 ± 50K for the rest; see Sect. 5.4). The weighted fit to the data points (red line) shows that the most massive cores tend to harbor a more extended emission of methyl formate.