but generally high bit rates and small files, seems to be about 2 MB / minute of audio.
The bitrate in kbps is kilo_bits_-per-second. There are 8 bits in a byte, so divide by 8 to get the file size in bytes-per-second. (That doesn’t include embedded album artwork or other metadata that adds to file size without changing the bitrate.)
Some websites say m4a is a smaller file size and higher sound quality compared to mp3. Is that true?
M4A (AKA MP4 or AAC) was designed to be the successor to MP3 so it’s supposed to be better. But here’s the thing… If a 256kbps MP3 is transparent (sounds identical to the uncompressed original), which it often is, a 256kbps M4A can’t be "better’ and a 360kbps MP3 can’t be “better”. (Technically/mathematically they are all lossy and every sample gets changed.)
And, if there are audible compression artifacts, at something like 256kbps, those artifacts often don’t go-away at higher bitrates (but the artifacts might not be present in M4A).
M4A is also supposed to be more tolerant of multiple generations of lossy compression. If you think about it… Opening an MP3, editing it, and re-exporting it at the original bitrate, doesn’t require any additional audio information to be thrown-away so the additional damage is a side-effect of the encoding.
It may also be easier to get gapless playback with M4A.
There were some “quirks” recently with iTunes and M4As (made with Audacity/FFmpeg). See [u]this thread[/u].
And, there are some “difficulties” setting the bitrate with FFmpeg. (I don’t remember the details at the moment, but I think there is a work-around.