iOS 9 is just released today, like very 16GB iPhone user it is time to find / delete things to make space for Installation.
I backed up everything to iCloud first, and cleared 1.8GB of free space in order for Whatsapp to backup itself. After Whatsapp successfully backed up, I had 1.8GB of space left. I checked moments later and I only have 800MB left, and then slowly rises back to ~1.2GB after an hour. I waited a few hours more, ( iOS 9 refuses to download as you may have already know), the free space never went back to 1.8GB, it stayed at 1.2GB
After i download and Installed iOS 9, I actually had 2.3GB of free Space. Since I knew I had the original 1.8GB of free space, i thought iOS 9 and some App thinning magic makes that 500MB extra.
But after an hour the Free space went back down to 1.4GB. Where did all that free space gone?
*Note I have I iCloud Photos turned on to space saving, but as far as I can tell the space for Photos has been a constant number.
*I know iOS9 will delete Apps to free space during installation and download them back later, but i am pretty sure that is not what's happened here.