iOS 9 "app slicing" feature delayed due to iCloud bug
It seems that one of iOS 9's interesting, not to mention space saving, features didn't make it to the first release after all. Apple announced to its developers that app slicing, known as app thinning to some, is currently unavailable in the current iOS release. This is due to an issue with apps' iCloud backups that breaks the feature. The good news is that Apple isn't giving up on the feature just yet and will re-enable it in a future update for iOS 9.
App slicing is a little known new feature in iOS 9 that didn't get much attention when the OS was announced. This feature allows developers to save users some space on their Apple device by only installing the app assets that are applicable to a certain device class. iOS apps that work on both iPhones and iPads have assets like images and and even code or whatnot that are sized and optimized to the specific form factors and hardware. Normally, a universal app such as this will install all those assets, regardless if you're installing only for an iPhone. This naturally takes up more space than necessary.
App slicing then will only download the assets that are applicable to the specific device. This can amount to significant space savings for an iPhone since iPad assets are mostly larger than those for the smartphone. Especially when you consider the 16 GB iPhone 6s, that's a significant benefit. App slicing would also ensure that devices with 32-bit processors need not download code or binaries built for 64-bit ones.
Sadly, things aren't meant to be so, at least for a while. A bug in the iCloud app backups seems to have the reverse effect. If a user tries to restore his or her apps on a new device, iCloud would install only the assets and data from a previous device without the assets for the one. This meant that a user moving from an iPhone 5 to an iPhone 6 or 6s would be given apps that are meant for the older, smaller, 32-bit model without any of the assets and files for the iPhone 6.
Apple promises that app slicing will make a return in a future software update but makes no promises when that will happen.
SOURCE: Apple