You *could* be wrong, but there's no easy way to tell. Let me illustrate:
Let's say you use a 6 Month Retention with unlimited Versions and daily backups and you had 100 GB of files to back up (and we'll assume no compression or block level backups in this example). Your storage is now 100 GB. You change 1 GB of files a day for the first 3 Months and you back up 1 GB * 90 or 90 GB. You then start changing 10 GB of files a day for the next 3 months for 10 GB * 90 Days or about 900 GB. . Your total storage is now 100 90 + + 900 or about 1.09 TB.
You then change your retention to 3 months and what are you left with? About 1 TB as you only removed the oldest files which accounted for less space than the newer backups.
The way to avoid this if it works for you is to limit the number of versions. In the Retention Policy in the backup wizard, check the option to keep a number of versions to a number that works for you. If you change a file every day and do not need 90 backups of that file, then change it to a number that works for you personally or your business.
Also, if you're using Block-Level Backups, then you need to make sure both the blok-level and incremental backups are scheduled on the Schedule tab (Recurring Pre-Defined or Recurring Advanced)..If you're not using block-level backups, then ignore this.
Also, keep in mind that if you manually unchecked a folder for backup that was previously a part of the backup plan, the backups for those files remain in storage indefinitely. If you no longer need them in backup storage, you should remove them using the Backup Storage tab in th product.
If you still have questions, please let me know.