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We present new Hubble Space Telescope imaging of the outer regions of M87 inorder to study its globular cluster (GC) population out to large galactocentricdistances. We discuss particularly the relationship between GC effective radii$r_h$ and projected galactocentric distance $R_{gc}$. The observations suggesta shallow trend $r_h \propto R_{gc}^{0.14}$ out to $R_{gc} \sim 100$ kpc, inagreement with studies of other giant elliptical galaxies. To theoreticallyreproduce this relationship we simulate GC populations with variousdistributions of orbits. For an isotropic distribution of cluster orbits wefind a steeper trend of $r_h \propto R_{gc}^{0.4}$. Instead we suggest that (a)if the cluster system has an orbital anisotropy profile, where orbits becomepreferentially radial with increasing galactocentric distance, AND (b) ifclusters become more tidally under-filling with galactocentric distance, theobserved relationship can be recovered. We also apply this approach to the redand blue GC populations separately and predict that red clusters arepreferentially under-filling at large $R_{gc}$ and have a more isotropicdistribution of orbits than blue clusters.

Globular Cluster Scale Sizes in Giant Galaxies: The Case of M87 and the Role of Orbital Anisotropy and Tidal Filling