iOS 8 requires your device to have at least 4.58GB free space before the upgrade. This is probably one of the main reasons the installation rate for iOS 8 were slow for several months following launch. Many users especially those with 16GB devices were having difficulty freeing up that much space.
Apple has significantly reduced the space requirement for iOS 9 upgrade to a mere 1.3GB. In addition, developers using iOS 9 beta have discovered a new space management feature which is useful for uses who have little space during the upgrade.
When you upgrade to iOS 9 on a device with limited space, a popup will offer the option to temporarily delete some apps on your device to make room for the upgrade. Apps that are deleted will then be reinstalled automatically after the update is completed.
This is great news for all users and will greatly spur the adoption rate. iOS 9 is compatible with all devices running iOS 8 and is scheduled for release this fall.