Many planetoids are airless but despite the lack of a collisional atmosphere, these bodies can experience volatile migration towards the polar regions via the exosphere. Volatiles can then be sequestered at the poles as surface ice cold-trapped in permanently shadowed regions (PSRs). Although cold-trapping plays a role in surface mass distribution and can help to produce transient atmospheres under extreme conditions, there is a lack of parametrical studies that focus on cold-trapping as a general planetary process, as opposed to within the context of specific bodies. Therefore, we run a Monte Carlo simulation to determine the water cold-trapping efficiency for planetoids of varying size, bulk-density, and insolation. We find that small planets lose most of their water via thermal escape while large planets favour photolysis. For a given bulk density and solar irradiation, we find the optimal planetary size where cold-trapping is most efficient. Surprisingly, the Moon is perfectly sized for maximum water cold-trapping given its composition and proximity to the Sun.