Island birds may become flightless, which lifts constraint on maximum mass.
Though that occurs in other contexts as well: Antarctica (hardly small) with penguins, Africa (dittos) and ostriches, Australia (again) with emus, and South America (again) with the rhea.
There are also flightless island birds, such as the Guam Rail, Henderson crake, Inaccessible Island rail, and across multiple islands, the cassowary.
