Model fits to our mass-metallicity data. Left: Centaurus. Right: Hydra. Compared to the model in Fig. 8, we include stellar evolution mass loss and dynamical mass loss according to Eq. (7), and then determine the best-fitting star formation efficiency. The dotted horizontal line represents the critical mass below which the clusters will undergo colour changes due to dynamical mass evolution. Below this line, asterisks indicate the data points without colour changes. Top panels: primordial half-light radius r_h is fixed at 1 pc. We find best-fit star formation efficiencies of 0.65 for Centaurus and 0.50 for Hydra and pre-enrichment values of (− 1.65, − 0.55) for Centaurus and (− 1.55, − 0.50) for Hydra. Bottom panels: the mass-radius relation as in Eq. (9), normalised to a mean r_h = 1 pc across the GC present-day mass range [5 × 10⁵: 4 × 10⁶] . We find best-fitting star formation efficiencies of 0.42 for Centaurus and 0.36 for Hydra, and pre-enrichment values of (− 1.70, − 0.55) for both Hydra and Centaurus.