Time machine space needed for backup is incorrect.

Configuration: new 27" iMac 2017. Mojave 10.14.5

System has 2TB Fusion drive partitioned 532 GB for Mac and rest for Boot Camp.

Actually using about 58 GB in the Mac and 900+ GB in Windows Boot Camp.

3 TB Toshiba attached via USB, and is listed in the Time Machine Exclusion screen.

Time Machine seems to have worked correctly from system setup on June 25, 2019 until today July 9, 2019 (15 days). Now getting popup with Backup Failed There isn't enough space on NAS.local. Detail screen:

Time Machine couldn’t complete the backup to “NAS.local”.

The backup disk needs 850.97 GB for the backup but only 338.23 GB are available. Select a larger backup disk or make the backup smaller by excluding files.

Latest successful backup: July 7, 2019, 10:17 PM

Now why would it need 851GB of space to backup 58 GB of Mac. The hidden partitions on the Mac only take up another 11GB, so call it 70 GB.

I have physically disconnected the Toshiba but it made no difference. Every thing else seems to be working ok, both Mac and Windows. I did install Paragons NTFF for Mac on July 5. However, no drives are mounted with it and have "Quit" the app via the menu bar. Paragon will be my next stop if nothing turns up here.

Any solutions or ideas out there?

Screen shots: