The iOS 9 OTA (over-the-air) update will take up much less storage than previous updates, as noted in the WWDC 2015 keynote address. Apple, using new technologies such as App Thinning, which lets devices download only the resources they require, was able to downsize the iOS 9 update from 4.6GB to 1.3GB. iOS 9 will officially launch in September, probably alongside the next generation iPhones. See here for a list of devices compatible with iOS 9.
How big will the iOS 9 update be?