This mashable article echoes the temporary 5.7-5.8 GB requirement for OTA updates.
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mashable.com/2014/09/17/ ··· ech-linkIt's not that iOS 8 occupies a ton of storage on your iPhone or iPad (iTunes says the file is about 1.2GB), but rather that the download itself needs up to 5.7GB of free storage to be completed.
And, as others have mentioned in the iOS8 thread:
But before you start deleting everything you've ever saved, there's a very easy solution: Sync your iPhone or iPad to the computer and download iOS 8 directly from there.
If you're taking this alternate route, be sure to select "check for update" when the prompt pops up on iTunes %u2014 not "restore iPhone." The latter option will indeed wipe all content from your device.
And yes, cynicism abounds.
If anything, the 5.7GB requirement to update to iOS 8 is a subtle nudge from Apple that you may need to splurge for the 64GB device next time.
Subtle?