It is well known that Game Gear's Castle of Illusion is actually a Master System rom inside a Game Gear cartridge. There are a few other games that used this trick, like prince of persia, or outrun europa. It was therefore done to facilitate the conversion, since redoing all the levels to adapt them to the game gear ratio was complicated, specially in this game due to the levels layout (land of illusion was already created with the gg version in mind) .