audio – How to encode silence efficiently with aac?

I used Audacity to created a WAV file with 4 samples of 10 seconds separated by 5 minute gaps of silence each. While the original samples were only a few MB the exported WAV file is 150 MB.

Encoding this WAV file with ffmpeg -i file.wav file.format reduced its size to:

file.aac/m4a: 30 MB
file.opus: 3 MB

To my surprise encoding the opus file to aac resulted in

file.opus.m4a: 3 MB

I am wondering why and how to achieve this small file size directly without an intermediate opus file.

I noticed that ffprobe file.opus.m4a shows a bit rate of 15 kb/s and ffmpeg -i file.wav -c:a aac -b:a 15k file2.m4a also results in a 3 MB file. However, the audio quality is miserable while it is excellent for file.opus.m4a.