How can I extract the aac audio from an mp4 file?

I tried with ffmpeg and -acodec copy but if i use mp4 as output it will still encode the video and I get the same file size.

If I use m4a as output it will somehow still encode the video and I get almost the same filesize.

With aac as output I can't open the file in puddletag although the file size suggests the video was stripped

Thanks in advance