Based on recent results on the frequency of Mg II absorption-line systems in the spectra of QSO behind RCS clusters (QbC), we analyse the effects of the cluster environment on the sizes of baryonic haloes around galaxies. We use two independent models: (i) an empirical halo occupation model which fits current measurements of the clustering and luminosity function of galaxies at low and high redshifts and (ii) the GALFORM semi-analytic model of galaxy formation, which follows the evolution of the galaxy population from first principles, adjusted to match the statistics of lowand high-redshift galaxies. In both models, we constrain the Mg II halo sizes of field and cluster galaxies using observational results on the observed Mg II statistics. Our results for the field are in good agreement with previous works, indicating a typical Mg II halo size of rMg II 50 h−1 71 kpc in the semi-analytic model, and slightly lower in the halo occupation number approach. For the cluster environment, we find that both models require a median Mg II halo size of rMg II < 10 h −1 71 kpc in order to reproduce the observed statistics on absorption-line systems in clusters of galaxies. Based on the Chen & Tinker (2008) result that stronger systems occur closer to the Mg II halo centre, we find that strong absorption systems in clusters of galaxies occur at roughly a fixed fraction of the cold-warm halo size out to 1 h−1 71 Mpc from the cluster centres. In contrast, weaker absorption systems appear to occur at progressively shorter relative fractions of this halo as the distance to the cluster centre decreases. These results reinforce our conclusions from Lopez et al. and provide additional independent support for the stripping scenario of the cold gas of galaxies in massive clusters by the hot intracluster gas, e.g. as seen from X-ray data.