Lots of things. Most companies now have huge development teams and that means that you're going to get a lot of duplicate and triplicate media assets. Duplicate and triplicate static libraries and frameworks (which Apple is trying to address with it's new dyld improvements[1]). As you also can't forget all of the analytics and A/B testing frameworks that are put in place. Each one of these apps can actually probably run 100^2 permutations of the app.

[1] - https://developer.apple.com/videos/play/wwdc2017/413/