ffmpeg produces different file sizes between MacOS and Linux

I have several .wav files. When I use ffmpeg to convert to m4a, it produces different file sizes between the two systems.

Example:

original.wav (on MacOS) – 418004 bytes
original.wav (on Linux) – 418004 bytes

Method 1:

for f in *.wav; do ffmpeg -i "$f" -c:a aac -b:a 96k "${f%.wav}.m4a"; done

MacOS – 18408 bytes
Linux – 15814 bytes

Method 2:

for f in *.wav; do ffmpeg -i "$f" -c:a aac -b:a 192k "${f%.wav}.m4a"; done

MacOs – 36428 bytes
Linux – 26060 bytes

Method 3:

for f in *.wav; do ffmpeg -i "$f" -c:a aac "${f%.wav}.m4a"; done

MacOS – 24168 bytes
Linux – 19765 bytes

As you notice, the Linux examples are quite smaller than the MacOS examples.

What is going on here that would make the difference? Is it a different AAC codec being used in both cases or is it the ffmpeg version? If so, what would the versions be doing differently here?

On my Mac, it appears I am using ffmpeg version 7.1.1
On Linux, it appears I am using ffmpeg version 4.3.8-0+deb11u3