Larger hulls means better accommodation, more space for helos, fuel, stores, equipment etc. I won't deny that there may be an element of prestige thinking also for selecting a larger (especially "largest yet" etc) design for a particular service, but given the distances these hulls potentially travel and where they are expected to remain on station for a while, such as all the disputed areas in SCS, I don't think 10k tons is a massive overshot. Anything between 5k and 10k tons is probably the new normal in coast guard OPVs, echoing the size growth for naval hulls for many of the same reasons. Steel is cheap.