What is the size of a typical iPhone backup to iCloud?
If you use iCloud Photo Library, then your photos won’t be uploaded to iCloud for backup. The same goes if you use iTunes Match and MUSIC: you music won’t need to be uploaded to iCloud for backup.

If you don’t use those services then yes, your iCloud backup will contain your photos and music. Otherwise you wouldn’t be able to restore your device the way it was.

And yet, even though I’m an iTunes Match and iCloud Photo Library user, my fairly barebones iPad takes up 1 GB of storage for backup.

My iPhone has a few more apps, (2 pages’ worth). However under iCloud → Storage, only one app stores over 200 MB (Runkeeper) with everything else taking under 50 MB. The vast majority takes up mere kilobytes. I don’t backup my Health data because this seems to be causing issues. Somehow my iPhone’s backup takes up 2.2 GB.

As far as data consumption, I believe iCloud performs differential backups, because while the initial backup takes a significant amount of time, subsequent backups seem to complete much more quickly.