If you inspect your NTDS folder on your Active Directory domain controller (the default location is %SystemRoot%\NTDS), you may notice that many EDBxxxxx.log files of ~ 10MB have not been deleted.
On my Active Directory domain controller, these files correspond in date/time to the System State portion of my nightly backup, tip 2265.
I have no idea why the files are not being deleted, but to resolve the problem, I added a line to the batch that invokes DELOLD:
DELOLD %SystemRoot%\NTDS\EDB0*.log 2
where 2 is arbitrary.