Sam Wilson wrote:

We'd say "in Mallorca" (actually we'd say "in Majorca" but we'd
pronounce it as if it were a German word!). We'd say "in Ireland"
too. We generally reserve "on" for islands you can see most of at the
same time: on the Isle of Wight, on Arran, on Lindisfarne, on Barra.
We might use "in" for islands that are also states of some kind so you
could use either "in" or "on" for the Isle of Man or Jersey. The
phrase "in the island of Ireland" is common but then so is "on the
island of Ireland".

Size is irrelevant IMO. I find that if it's a country, you say "in",
otherwise you say "on". I would certainly say "on South Island" (NZ), "on
Baffin Island" and "on Hokkaido", even though these islands are way too big
to see the whole thing at the same time.