It appears that all of my backup-to-disk jobs are all keeping multiple full backups on the disk - I can certainly see the logic, but it has filled up the storage array being used for this purpose.
In my investigation and testing, I started looking at the job.options.BackupStorageOptions object. It has a BackupCycles and a RetainDays value in it.
To me, a "Backup Cycle" would be one pass of a full backup, thus a job with a BackupCycles set to 7 would keep 7 full backups plus the incrementals in between. Is this understanding correct? Is the RetainDays for identifying how long to keep incrementals?
I ask because after setting it to 1, it changed the "restore points" from 7 to 1 within the wizard. I would have taken restore points to include incrementals. If my understanding of that was flawed and it is indeed full backups, I might suggest changing the label from restore points (of which 7 seemed logical... day 1 is the full, days 6-7 would be incremental, rinse and repeat) to "Backup Cycles" or similar.
I'm running a job on a small VM now to see what changing the BackupCycles does, but until it finishes and just in case the results are inconclusive, I thought I'd turn to the forums.