I have some music downloads which are in the form of .m4a files and I want to cut them up using ffmpeg.

I find that something like

does the trick but I have the problem that I am unable to put tags on the .aac file. Easytag for instance will not write to them so I need to convert to mp4.

ffmpeg does seem to able to output directly to mp4 but I found that my software does not behave proerly with these files. What does work is going from aac to mp4.

Now I come to the real question. The aac filesize is slightly larger than the mp4 filesize. If mp4 is a container for aac why should that be?

Is my approach correct?

The mp4 format is used when you have both video and audio stream.

If the file size of the mp4 is slightly smaller, i guess it strips some excess metadata off the aac stream.

As I understand it the container for aac is mp4 but by convention if it just contains audio it is usually renamed as m4a.

My only reason for using mp4/m4a rather than just aac is that I cannot find any other way to write the tags.

Out of curiosity I did a net search, this was the first hit, but there was more.