Andrew Cunningham, writing for Ars Technica:
Apple already talked in the keynote about how it had reduced the amount of space required by the iOS 9 OTA update from around 4.6GB to 1.3GB, but a more transformative technology only got a passing mention: App Thinning. In short, apps in iOS 9 will leave your phone or tablet with more free space in the first place.
My fiancée has been using a 16GB iPhone 5S for almost two years and has definitely felt hindered by its limited storage space. She often has to pull all the photos off of her camera roll, make tough decisions about what music to sync from iTunes, and delete apps from her device to give herself some breathing room. If App Thinning does what it claims it’s going to be a big deal for iPhone owners with 8-16GB of storage.