I know that (it still does consume more than iOS 7 did, but not 6.9 GB of course). But it's pretty inconvenient to have to delete more than half of the contents of my device for the update.

Then don't update over-the-air. If you plug your iPhone into your computer and update via iTunes you don't have to delete anything from your device.

Well, 1.1GB for the file, and 5.5GB for the installation.

That's still huge.