Top: effective radius r_eff vs. age t for young massive bound star clusters (YMCs) in the Milky Way, the Local Group, and external galaxies which are distinguished by different filled symbols. The symbols are colour-coded according to the clusters’ respective photometric mass, log ₁₀(M_phot/M_⊙). These observed data are given in Table 1. Overlaid in the panel are the computed curves for the evolution of projected half-mass radius (or effective radius) r_eff(t) for model star clusters with initial masses M_cl(0) and half-mass radii r_h(0) as given in Table 3, which do not include a residual gas expulsion phase. These curves are distinguished according to the legends in the panel where “PB” indicates that the computed cluster includes a realistic primordial binary population (see Sect. 2.1). These lines are also colour-coded according to the corresponding clusters’ instantaneous total bound mass log ₁₀(M_cl(t) /M_⊙). As can be seen, if the clusters evolve from compact sizes determined by substructures in molecular clouds, their secular expansion substantially falls short of the observed sizes of YMCs (see text). Bottom: here, the curves and the colour-coding are the same as above except that the data for the young massive associations in Table 2 are plotted.