Incremental Backups Have No Real Mail

[the_griz]

I have a zimbra mail store that is 146GB when backed up in full. When the incrementals are run, they are 1-2MB! With several thousand messages coming in each day, 1-2 MB tells me that incrementals are simply not working. How can I troubleshoot to determine what is wrong?

Backup is run out of the cron.
[zimbra@mail ~]$ zmschedulebackup -q
Current Schedule:
f 0 1 * * 3 -a all
i 0 1 * * 0-2,4-6
d 6d 0 20 * * *

full on Wednesday with incrementals the rest of the week.