Shoutbox

It won't be anything to do with limits on the log size. It's all to do with file locking.

If a log file is locked, for whatever reason, Messenger will create a new file to continue logging to. Then Messenger will always use the newest file for future logs. This is why, when you move the new files out, Messenger goes back to the original file: as far as it's concerned, the original is now the newest.

Edit: Plus! logs can also suffer from the same problem.