I have a big iPad app that has an iPhone & Watch "remote" app, ie. a little app that runs on the iPhone/Watch to remote-control the ipad. All of this is together in a universal app. For app thinning I've separated assets in the assets catalog as required. The app is already on the App Store (released). The app has a minimum SDK version of iOS 8.1.
When I take the Archive that was released on the App Store, do an adhoc export, choose to build from bitcode, and choose to export for iPhone 6S, then the App Thinning Size Report.txt shows "19MB compressed, 39.4MB uncompressed". 19MB would be acceptable to me, considering there's 2 * swift dylibs in there (watch appex, and main app)
BUT: On the app-store when viewed on an iPhone 6S with iOS 9.3.4, the app size shows as a massive 42MB download! I assume this is compressed, so why is this more than twice as big as the 19MB that I was expecting? Is it because the app supports iOS 8.1, and therefore no app thinning ever happens even for devices that have iOS 9?
I want to release an update to fix this but I don't know where to begin.
I burned one of my 2 yearly Apple TIs for this. Turns out that what you see when you're on the App Store, looking at an app's size, is it's uncompressed/installed size. Not its download-size.
Also, on itunes connect it turns out you can click on the build (or go Activity -> All Builds -> (choose build) Build Details), and then there's a link there called App Store File Sizes. When you click THAT, you get much much more detail - ie. both sizes for each device type.
So, everything is awesome. Everything is cool when you're part of a team.