A promised iOS 9 feature called "App Slicing" (also known as "App Thinning") finally goes live, but you must be using iOS 9.0.2 or higher.
The feature, which is meant to reduce the amount of storage space apps take by delivering optimized packages for specific devices, was put on hold so Apple could fix a bug relating to iCloud backup.
Here's how Apple describes "App Slicing":
"Slicing is the process of creating and delivering variants of the app bundle for different target devices. A variant contains only the executable architecture and resources that are needed for the target device. You continue to develop and upload full versions of your app to iTunes Connect. The App Store will create and deliver different variants based on the devices your app supports. Image resources are sliced according to their resolution and device family. GPU resources are sliced according to device capabilities. For tvOS apps, assets in catalogs shared between iOS and tvOS targets are sliced and large app icons are removed. When the user installs an app, a variant for the user's device is downloaded and installed."
Now that bug has been fixed users of iOS 9.0.2 and higher will receive device-specific versions of apps, saving both bandwidth and storage space. Those using earlier version of iOS will continue to receive what Apple calls "universal apps."
If you're running a 16GB iPhone or iPad then this should help buy you more time by freeing up some much-needed storage space.