The Small Island

If the island is small enough to have upon it only one town and if the island is named for that town, then it is small enough to qualify for any rule about "cities, towns, and small islands." So, these rules are specifically rules about population centers. In fact, there is a reflex of the rules that state that you do not use a preposition with cities, towns and small islands: If you do use a preposition with a city, town or small island, you mean "the vicinity of", in other words, the preposition marks the place as "town plus environs." Thus, in Romam means "into the area around Rome" and ad Veronam means "toward the area around Verona."