iOS 9 is Better for iPhones with 16GB of Storage
Apple continues to sell a 16GB iPhone 6 and with changes to iOS 9 and comments from executives likely will keep selling a 16GB iPhone 6s this fall. The iOS 9 update is more suited for phones with small storage sizes.
The iOS 9 update is smaller than the iOS 8 update so it will allow users to upgrade easier, without deleting apps, videos and photos.
Another feature is App Thinning. This is a fancy way of saying that instead of downloading the version of an app that will work on any device, iOS 9 will allow your iPhone to only download the parts that it needs to work on that specific iOS device so there is less wasted storage and apps are smaller overall.