iOS App Store for iOS 9.0 provides significant optimizations that allow users to download only the assets their device supports and to lazily download assets or data
The App Store and operating system optimize the installation of iOS and watchOS apps by tailoring app delivery to the capabilities of the user’s particular device, with minimal footprint. This optimization, called app thinning, lets you create apps that use the most device features, occupy minimum disk space, and accommodate future updates that can be applied by Apple. Faster downloads and more space for other apps and content provides a better user experience.
This chapter describes the three components of app thinning: slicing, bitcode, and on-demand resources.
Read more from the source: developer.apple.com