I don't know about iOS, but when I look at big APKs on Android, what I usually see is a good number of images that are essentially repeated 3-4 times for different dpi ranges, and localized text, in utf-16, for many languages, uncompressed. I think I saw something recently about Google providing tools to compressing localized text, but I may be misremembering. The logic for storing localized text uncompressed is something about mmaping the data and accessing by offset for efficiency, but when an app supports a majority of the world's living written languages, and users tend to support only one or two, the storage cost doesn't make sense. At one point, Google Play tried to do localization packs, but it never actually made sense to use, so nobody does. I know proprietary options exist to compress localized text in apks, but being part of the default system is important.

</dated rant> (edit: my information is old, and perhaps hopefully outdated)