Ratio of the constrained initial to the present-day density as a function of the number of completed initial crossing-times, t_cr,initial, for seven clusters (filled squares, where USco-A is not shown for the same reason as it is not depicted in Fig. 5). Errors in the x- and y-directions follow from the error in ρ_ecl and error propagation. These clusters may originate from the bottom-left corner of the diagram, the region supposedly occupied by the molecular clumps shown in Fig. 6. The clumps are probably still collapsing and have not yet reached their peak densities, hence have a density ratio < 1. Filled circles indicate the constraints for Galactic GCs as in Figs. 5 and 6 (ages taken from Marín-Franch et al. 2009). The solid arrows indicate a possible evolutionary sequence. This evolution cannot be understood in terms of single-star N-body star cluster models without gas expulsion. These models are shown by the evolution of the M_ecl = 2 × 10⁴ M_⊙ clusters with the indicated initial-r_h and a Kroupa (2001) IMF between 0.1 M_⊙ and 1.2 M_⊙ on circular orbits around a realistic Galactic potential over 4 Gyr (Küpper et al. 2010, solid black curves). Residual-gas expulsion and/or energy generation through binary-burning probably needs to be invoked to explain the expansion of the real clusters (see the text).