This is by design, so that if you email a favorite to someone, you don't inadvertently send a large megabytes+ file. Over a WiFi connection, it's not a big deal, but if you're using your phone's 3G/4G connection, this could use up your available data quota fairly quickly..
If you email a 3 minute long favorite (in ogg format), it should be approximately 750 kilobytes in size.
Out of curiosity, what are you using the 5 minute recordings for ? I'm just curious. It's fine if you don't want to answer :).
Kim