Just an observation...
It appears that my logfile in log/2007/12/ was deleted by the webalizer script when it created log/2008/01/. The old log file was well above 6GB at that time.
The new logfile in log/2008/01/ is already 200MB even though I have set the max size in ISPConfig to 5MB.
I found it interesting that the old logfile was indeed deleted. It almost seems like the webalizer script is unable to delete the logfile if the file is in use.