For comparison, he has also written an MP2 decoder, which is almost an order of magnitude smaller: https://keyj.emphy.de/kjmp2/
Note that MP3 patents expired not long ago, so the format is now "free": https://news.ycombinator.com/item?id=14240645
He also recommends checking this one that is a clean re implementation, not the extraction of FFmpeg code:
https://github.com/lieff/minimp3