Comparison between the simulations results for 30% (dotteded), 60% (dashed) and 70% (solid) SFE for model CK 1 and the observed properties of young massive compact clusters (circles). a) shows the relation between cluster age and size r_c; b) the cluster mass; and c) the cluster density as function of cluster size, respectively. The colours indicate cluster age (red t_c < 4 Myr, green 4 Myr < t_c < 10 Myr, blue t_c > 10 Myr) where in the simulations it was assumed that the cluster had an age of 1 Myr at the time of instantaneous gas expulsion.