When it comes to packaging a game with an engine like Unreal Engine, there is not much that you can do to reduce the final installer file size, as there are so many files, headers, and things related to the engine itself that need to be shipped along with the game. Still, here are some hints that could help in reducing the final game size:
Using less textures and game contents will guarantee that you have a smaller content folder within the IPA.
Any unused content must have no reference. For example, if there is a testing level and it has testing content, then it needs to be removed from any logic; because any content referenced in the game will get cooked into the IPA.
Switching the project to C++ is a great way to reduce the size a little bit, because you'll be able to disable some of the plugins such as Slate. Normally, the project cannot make any assumptions about which plugins and libraries you are using.
Zipping the final IPA file before submitting to the App Store...