I am having problems with my synthetic fulls bloating to ~3x their expected size.

My total data to backup is ~250GB (compressed), but my fulls are now ~750GB in size.

I can see that during the synthetic full backup it copies close to 750GB of data, then adds the a small amount of additional data for the current month.

I'm thinking maybe it's my retention settings, but I'm not sure why.

My intention for retention is to keep fulls & incrementals for the last 3 months.

Here are my retention settings:


The reason I specify 3 months for 'Keep files for' AND 3 months for the monthly GFS settings is that this is a dry run for enabling MSP360 managed Compliance Immutability. My understanding is Immutability functions based on the GFS settings only.

Based on those settings, what I'm expecting to happen is:
- Synthetic fulls pull ~250GB of required data from the last month's full backup and result in an ~250GB full backup
- Fulls and incrementals are kept for 3 months

To me,it's almost like the backup is interpreting my settings as 'Include the last 3 monthly fulls in every monthly backup, because you want to keep them for 3 months'.

My expectation is that it would achieve this by keeping the old monthly backup files on the storage as they are, not roll them all into the current month as well.

The only other factor in all of this is that I have a 90 day compliance object lock on the storage BUCKET, so maybe it's expecting to be able to delete something and it can't??

Can anyone see what the issue is here?