Data points and their colour-coding in both panels are identical to Fig. 3. The curves are the computed evolution of the projected half-mass radii (or effective radii) r_eff(t), including a gas dispersal phase with star formation efficiency ϵ ≈ 33% (see Table 4, top part and text for details). As in Fig. 3, the curves also follow the same colour-coding according to the corresponding clusters’ instantaneous total bound mass log ₁₀(M_cl(t)) (the same set of curves are overlaid on both panels). These figures imply that even if the YMCs evolve from filament-like compact sizes, such substantial (and explosive) gas dispersal will expand them to their present observed sizes (half-mass radii) in the Milky way and in the Local Group (top panel). However, to reach the sizes of the most extended Local Group YMCs, they need to evolve from r_h(0) ≳ 1 pc half-mass radii, unless such objects are low-mass cluster complexes (Brüns et al. 2009). The latter is also true for the young massive associations (bottom panel). See text for details.