So there’s a podcast I listen to often that’s more or less a live broadcast radio play. I have the back catalogue downloaded off YouTube. But there’s about ten minutes of ads at the beginning, a twenty minute break halfway through, and five minutes of outro music at the end which makes binging the episodes sort of a pain (especially since the ads are all many months outdated). So I decided to cut out those sections. Overall it means slicing out a half hour to forty minutes from each audio file. Yet despite a significant reduction in length, the exported files end up larger than the imported ones.
So for instance, the last episode was five hours and twenty five minutes long and the audio file was 297 mb. I cut out the breaks and ad spots. The resulting file is just under five hours long and yet it’s 319 mbs. What happened? Surely a shorter file should get smaller, not larger.
The size of a compressed file can be determined from the bitrate. kbps is kilo_bits_ per second. There are 8 bits in a byte so divide by 8 to get kilo_bytes_ per second. Then any embedded artwork will add to the file size.
For uncompressed audio formats (such as WAV) then yes it would be smaller (unless you changed the format, such as going from mono to stereo).
For compressed formats (such as MP3) there is a trade-off between file size and sound quality. Encoding (or re-encoding) to MP3 format always reduces the sound quality to some degree. The lower the “kbps” the smaller the file but more sound quality is lost. Higher “kbps” settings lose less sound quality but at the expense of larger file size.
This web page can tell you what the “kbps” of an MP3 file is (plus a lot more information about the file format): MediaInfoOnline - MediaInfo in your browser
Audacity’s MP3 settings are documented here: MP3 Export Options - Audacity Manual