Complex sentence: The latter is often referred to as the "mainland", especially by its residents, it being somewhat larger but having a lower population, although in general practice, the mainland refers to both North Island and South Island as opposed to the smaller offshore islands.
Split into simpler sentences: The latter is often referred to as the "mainland", especially by its residents, because it is somewhat larger (but with a smaller population). However, in general practice, the mainland refers to both North Island and South Island as opposed to the smaller offshore islands.