[FFmpeg-user] reduce file size: extract audio from mp4 (video)

[Beeblebrox]

I have several video files that I would like to convert to audio-only.

Is there a simple ffmpeg command to extract the audio as-is from the 
video file, AND achieve file size reduction? For example,

"$ ffmpeg -i vidoe.mp4 -vn -acodec copy audio.mp3" naturally fails with:
"[mp3 @ 0x80d03f020] Invalid audio stream. Exactly one MP3 audio stream 
is required. Output #0, mp3, to 'audio.mp3':
Metadata:  major_brand     : isom, minor_version   : 512, 
compatible_brands: isomiso2mp41, encoder         : Lavf55.19.104
Stream #0:0(und): Audio: aac (mp4a / 0x6134706D), 44100 Hz, stereo, 99 kb/s"

Other methods advised by google mostly have bitrate-type solutions
"$ ffmpeg -i video.mp4 -vn -ar 44100 -ac 2 -ab 192 -f mp3 music.mp3"
I have tried some of these, but the resulting files are larger than the 
original mp4, which makes absolutely no sense.

Regards.