iCloud backup storage size

The size of the backup on my phone has more than doubled without storing any additional information on my phone and now I can not backup to icloud. I would buy more storage but there is no reason to do so since I haven't added any additional files or photos. I did have to sign out of my icloud account and back in and that is when I noticed the size of the backup doubled. What can I do to correct this so I don't have to buy more storage?