When I copy one side of an audio cassette onto a SanDisk, it’s 45-55 MB size file. After labeling the songs in Audacity the resulting *.aup file is 800-900 MB, 20x larger!? Labels are KBs, not MBs. Roughly, for every 50 MB of music added I’m losing a GB of storage. Doesn’t make sense. Solutions? Thank you. – JRB
If by labels you mean embedded metadata or “tags” (title, album, artist, etc.), try MP3TAG. It adds the metadata without touching the audio. (It works on all popular formats, not just MP3.)
AUP3 Audacity Project files use a high-quality uncompressed format and they hold “undo” and overhead information. They can be HUGE! (And you shouldn’t be using them on an external drive.)
If you export as the same format it will be the same size.
If you want to check the format before & after, try MediaInfoOnline (Or you can install MediaInfo on your computer.) It probably won’t recognize an AUP3 file. I’ haven’t tried it.
You may not need to make an Audacity project… I rarely do.
With MP3 or other lossy compression the file size is proportional to bitrate. Bitrate (kbps) is kilobits per second. There are 8 bits in a byte, so divide by 8 to get kilo bytes per second.
CD audio is uncompressed, 44.1kHz, 16-bit, 2-channel stereo so that works-out to 44.1K x 16 x 2 = 1411kbps or 176k bytes per second, or about 10MB per minute.
Any embedded album artwork (which Audacity doesn’t support but MP3tag does) is not included in the bitrate so the file is larger than “calculated”.
Thanks for the quick reply. It mostly went over my head but I checked those links and re-read your email a few times. I thought describing my process in detail might help, and in doing so I discovered the file explosion comes when opening in Audacity. The cassette player saves side 1 of a C90 cassette as a 46 MB “MP3 audio” file on a SanDisk. When I open or save in Audacity on my MacBook it becomes a 738.8 MB Audacity Project (*.aup3), before doing anything to the file. I can play the SanDisk MP3 file in Music (nee iMusic) but cannot (?) edit into individual songs/tracks, hence Audacity. That you say a CD is 10MB per music minute suggests I’m stuck (e.g., my Handel’s Messiah CD is 1.42GB). But I’m still baffled how 46 MB becomes 739 simply by saving in another format. In digitizing my analog music collection I’ve gone thru two turntables and 5 cassette players and they ALL use Audacity. Virtually all my cassette are home-made, recorded off the air from various Alt-Rock radio stations, so there’s nothing embedded. Is there an editor that will keep it close to 46MB instead of 739MB?! Thanks again. – JRB
OK… Let’s try again…
“Just for fun”, check the original format on the SanDisc with MediaInfo.
If you export as the same format it will be the same size. 
You gave us the file size without telling us the playing time or the format…
File size depends on bitrate. I showed you how to calculate the bitrate & file size for uncompressed files. For compressed files it depends on the compression settings.
With lossy formats like MP3 you can choose a wide range of bitrates, with higher bitrates generally giving better quality.
With lossless compression (FLAC or ALAC) you are more-limited and you can’t choose the exact bitrate you want. The files end-up smaller than the uncompressed size and larger than the lossy compression size.
When I open or save in Audacity on my MacBook it becomes a 738.8 MB Audacity Project (*.aup3)
That’s only temporary and if you may not need an “Audacity project”. You can Export as WAV or MP3 etc.. instead of Saving a project, or you can do both.
The exported file will be smaller (size depending on the format) and you’ll need a regular audio format anyway because AUP3 only works in Audacity.
I will suggest FLAC at “CD quality” (44.1kHz, 16-bit, 2-channels). Or, 48kHz files are only about 10% bigger. And of course if you have some mono files they can be 1-channel mono which will play through both speakers and will be half the size if uncompressed.
CD quality is generally better then human hearing, and it’s better than vinyl or cassette so it’s usually good-enough for anything.
FLAC is lossless compression. The files are almost half the size as and metadata is better-supported than on WAV. It makes a good “archive format” even if it’s not your everyday format. Some players & player software doesn’t support FLAC but it’s lossless so it can be converted to any other lossless or lossy format anytime in the future.
A lot of people keep a FLAC archive (1) and use MP3 or MP4 on their phone or portable player.
Lossy compression like MP3 throws-away information (and uses some other tricks) to make a smaller file. But it’s “smart” and it tries to throw-away details you can’t hear anyway. The lower the bitrate, the more data is thrown way so higher bitrates generally mean better quality (to a point).
A good-quality MP3 is usually about 1/5th the size of CD quality WAV. On my antique iPod I generally use “VBR 0” which is the “best” variable bitrate setting. (2)
M4A (aka MP4 or AAC) is supposed to be better than MP3 and it’s almost as universal. But, you’ll need to install FFmpeg to export as M4A.
(e.g., my Handel’s Messiah CD is 1.42GB).
CDs only hold up to about 700MB. But once you open it in Audacity it’s 32-bits so that would be about right in Audacity’s temporary format or as an AUP3 file. And again, the AUP3 and temporary files will grow as you edit and it saves undo information.
(1) Make sure to keep an "archive’ or back-up (or multiple back-ups) so you don’t have to digitize again later.
(2) With VBR, the bitrate is adjusted moment-to-moment depending on how hard it is to compress. i.e. Silence and simple sounds or voice can use a low bitrate without hurting sound quality.
Sometimes when I think about it I am impressed all over again.
We regularly digitize and play back data that is sampled 40 thousand times a second or more. Even the cheapest consumer devices can handle it.
It’s astonishing when you ponder it.