iCloud backup size after iPhone migration
After talking to Apple Support and trying various things, I conclude that this is a bug in iCloud.

I've made the following observations:


Backups to iTunes always have the size they should have (about 2.8GB in my case)
Backups to iCloud when first created are of the correct size and then constantly grow (after I month I saw about 10GB).
Apple Support has no solution to offer except to periodically delete the iCloud backup and recreate it from scratch.


For reference, this only started after I migrated my iPhone 5S to an iPhone SE via an iTunes backup.  Before this migration iCloud backups worked as expected.

Edit in 2018: indeed the bug was fixed later and backup size went back to normal a few weeks after this answer.