iOS 9 and iOS 8 size difference Wired reported today that the new iOS9 software will only be 1.3BG in size. A great deal smaller than the 4.6GB of iOS8. 

My question is quite simple - how is this possible? Was there a great deal of redundant code in iOS8?

Source - http://www.wired.com/2015/06/ios-9-new-features/?mbid=social_fb
The space needed to download the first part of the installer was optimized. It's possible to engineer for any number of trade offs - in this case, work was done to reduce the footprint of the download. 

I haven't reverse engineered the installer, but my hunch is that iOS 8 added new features to allow better cleaning and update hooks so this is more a feature of iOS 8 which runs the new installer than 9 being so much smaller. I'm sure 9 is somewhat smaller, but this is a big jump as you have noticed.