From what I understand from the keynote, it's not the operating system itself that takes up less space, it's just the update to iOS 9 that takes up less space, right? Which is still a big thing for people who have less storage on their devices.
They also mentioned a feature called App Thinning, which allows iOS to download only the base app bundles from the App Store and load additional data on a per-device basis. Many apps, especially universal apps, bundle all the images and videos to cover all devices, wasting lots of space. I don't need iPhone 5 or iPad resolution images on my iPhone 6. This could have some tremendous impact on space usage if it works like I think it works.