Holy c**p! I just upgraded my 5s from 8.4.1 to 9.0 and my free space went from 4.85 GB to 1.67 GB! Yikes! Is iOS 9 really taking up that much more memory?
The size varies from 1 to almost 2 GB, depending on the device, but it shouldn't need that much space. Try resetting your iPhone, using Home and Power button. If it still needs too much storage space, restore it using iTunes. Make sure you have a recent backup before you restore.
My iPhone said it's 1.2 GB, but I've read somewhere else that their update size was 1.8. Still clearly less than iOS 8.
In the process of updating my wife's phone now. It's at 10.93 GB free before the update. We'll see what happens.
Did a hard reset on my phone and it made no difference. If I backup the phone to iTunes and then do a restore will I lose any settings or email account setups, or anything like that? Really don't want the headache. Thinking of getting a new 6s one of these days anyway. 64 GB this time.
Your settings will be included in a backup, so you don't have to remember everything you chose before. Here's what both backup methods store, and what's not included: About backups in iCloud and iTunes - Apple Support
Finished my wife's phone update. Now she's down to 8.8 GB free space from 10.93. So iOS 9 used an additional 2.13 GB over iOS 8.4.1. That's a pretty good-sized increase, seems to me.
And getting back to my original post, my upgrade used 1.05 GB more than hers did. I'm much more of a power user, whereas she's very minimalistic. So I can certainly see how my upgrade could be significantly larger, but that still seems excessive.
The 1.3 gb is how much it needs to update. That's not the end result. I hardly lost any storage space at all after updating iOS 9. It needs space to update, but after it's updated that space is available again. IOS 9 does not take 1 or more gb permanently.