The .old log files are never automatically compressed. If you rotate again, it will be renamed based on its last-modified date and only then will it be automatically compressed.
This is for two reasons. First, log entries come from many different parts of our network so there needs to be a "cooling off" period after rotating to make sure all parts of the network have switched to the new log. Otherwise, stuff might still be logged while compression is happening, and it would get lost.
Also, many people like to run analysis against a static log file. That way, if you change an analyzer setting and run it again, or if you want to do more than one thing to the log, you're working with a consistent set of logs. After log files are compressed, they're a lot harder to analyze, so keeping the .old file around often makes log analyzing much easier.
You're free to remove the .old log file yourself if you like, although we recommend waiting until you see log entries appearing in the new log file. If you rename or compress it yourself, it will not be auto-rotated in the future.