These databases keep track of signups and removals. They also keep track on messages sent, but more detailed tracking is in the sent logs for each mailing list.
Sometimes the log files can get out of hand. Use the reset all log files menu item on the main page to start your logs from scratch. This won't affect the sent logs.
You can select a date range and choose Purge Selection to remove the logs for the selected database within the date range selected.