Update: Read the iOS 9 review.
At WWDC, Apple's Craig Federighi proudly revealed that installing iOS 9 will require just 1.3GB of storage space on any supported iPhone or iPad. That's a huge improvement over iOS 8, an upgrade that caused major headaches for owners of Apple's entry-level 16GB devices. Those 16GB iOS products are still being sold, so Apple is going to even greater lengths to ensure smooth updates for iOS 9 and beyond. Per 9to5Mac, developers running the second beta of iOS 9 (released today) have noticed that the operating system, if necessary, will now prompt users to temporarily delete apps before applying an update to clear the necessary storage space.
The process is completely automatic, and once the update is successfully installed, iOS will reinstall those apps. 9to5Mac says that apps are restored without any loss of user data, though the dialog box in the screenshot above doesn't directly confirm this. Still, this new feature combined with iOS 9's smaller footprint should make keeping up with the latest iOS release easier than ever before. With iOS 9, developers are also gaining a new ability to "thin" their apps, freeing up even more space for your media, photos, and other content.
Apple WWDC 2015: iOS 9 features