Back in June, we wrote a bit about App Thinning, a collection of iOS 9 features that are supposed to make iOS 9 apps take up less space on iDevices. Apple has just announced to developers that one of those features, "app slicing," is not available in current iOS 9 versions due to an iCloud bug. It will be re-enabled in a future iOS update after the bug has been resolved.

App slicing ensures that your iDevice only downloads the app assets it needs to work. In older versions of iOS, all devices downloaded "universal" versions of apps that included all of the assets those apps needed to work on each and every targeted iDevice. If you downloaded an app to your iPhone 5, for example, it could include larger image assets made for the larger-screened iPhones 6 and 6 Plus, 64-bit code that its 32-bit processor couldn't use, and Metal graphics code that its GPU didn't support. That's all wasted space, a problem app slicing was designed to resolve.

Apple says the iCloud bug affects users who are restoring backups to new devices—if you moved from that iPhone 5 to a new iPhone 6S, for example, iCloud would restore iPhone 5-compatible versions of some apps without the assets required by the newer, larger device.

For now, Apple says that devices running iOS 9 will continue to download the universal versions of apps along with all their assets, whether they're needed by your specific device or not. TestFlight, the beta app distribution service that Apple purchased in 2014, will continue to distribute software tailored for specific devices, but regular users will need to wait for that iOS update before they begin to see the feature's benefits.