It's often the high-res assets (images, videos, sounds, animations etc.) which take up most of the space, not the code itself.

When it comes to the code, the reason that they are bigger is how apps are distributed and have isolated runtimes. There are several commonly used sdks and libraries but because of how mobile app distribution works, these common libraries have to be bundled with each app and are not installed as a reusable library (like you can do with Linux, for example).