Q: Icloud full but backup and apps using only 1/3 of my account

I receive notices that my 25 Gb iCloud Storage is almost full and has only 1 Gb free.  Yet when I go to manage storage, the backup of my various devices amounts to only 8Gb, and the information in my various apps only another Gb. Is it iTunes?  Photos (I only backup my photo stream so I do not need to download photos with a hard connection).  What am I not understanding?  Happy to get the additional storage if I truly need it, but if I can manage my data better, I rather do that.  Thanks for your help!