iOS 8 upgrades required over 4GB of free space in order to perform updates, which caused the adoption of the mobile operating system to lag behind iOS 7. With iOS 9, updates will only be about 1.3GB in size, but it can still be tough for many users to free up that much space, especially if they’re not syncing to iTunes to manage storage manually.
Apple revealed at WWDC that iOS 9 will require less free space to update now that devices don’t have to download and unpack software updates. There’s also new notification options for upgrades that let you update your device when you sleep or use it least.