Everyone was able to migrate to iOS 8. Some users didn’t have the memory management needed to migrate OTA because of the deflation requirements of the package.
Using iTunes allowed the computer to do the decompression, and a tethered migration could be achieved.
Current bit weight for a few devices I am testing for iOS 9b1 (this will change):
- iPad 3 1.9G
- iPad Air 2 2.26G
- iPhone 5 1.8G
- iPhone 6 2.1G
All supported iDevices have the capacity to migrate to iOS 9, and with app-thinning, slicing, photo space optimization and a little memory management know-how it will be a better user experience than iOS 8.
More iOS 9 specifics at the top link of the splash page: