I've used Audacity to edit m4a audio files and notice if I re-save it after edit, the new m4a file size is about 4 times bigger using the m4a export option using the export m4a(AAC) option. I suppose this is becuase the bit rate is higher and there is no option to change it on export menu? If I choose custom (ffmpeg) export option then the file size reduces to somewhere closer (if I enter lower bit rates) to original but output can be clearly heard to be less clear in sound quality.

So what am I doing wrong with Audacity and how can I recreate same m4a file size with same sound quality?

Why not save the files in the mp3 format? Unless you're using high end audio equipment you'll probably not notice any loss of quality.

Yes, thanks. I have resorted to saving to mp3. Previously I chose mp3 with a higher bit rate making files much larger than original m4a files, but if I choose a lower bitrate the file sizes can be about the same and I cannot tell much difference in sound quality.

Indeed if I save with 128kbits or 64kbits I cannot tell much difference in sound quality to my ears?

Would Audacity be a sensible choice for capturing Youtube music in MP3 format under Windows 8.1 or is there a better way, please?

I only use Audacity to edit files - I like it as its open source. However now I have switched to Adobe Audition which is a heck of a lot faster opening and saving files and overall seems more efficient on the cpu.

To get Youtube music I use a Youtube down-loader (mp4 file format) then convert it to mp3. Much faster than recording with Audacity or the like.