FLAC file size oddity

Hello Audacity experts,

I’ve made a video with the audio recorded in FLAC. The total file size, including video, is 29.6 MB. The entire thing is done with ffmpeg.

By opening the video file in Audacity, the FLAC track is automatically extracted. Nice!

If I immediately export the track to FLAC, the file size is 29.9 MB. Bigger than the original including video, and I haven’t even touched the track. (I should point out that the video file is actually quite small, about 2.9 MB, since it’s merely a slideshow.)

Now if I mess around with it by removing noise and normalizing, the file size jumps to 50.3 MB. But I haven’t increased the quality in any way.

Can someone explain this behavior?

As a bonus question for those of you who are familiar with ffmpeg: If I try extracting the FLAC file directly from the video using

ffmpeg -i video_file.mkv -acodec copy audio_file.flac

, the resulting file size is as expected 26.7 MB, but I only get the first 19 seconds of audio out of 18:53. What’s going on?

Have you considered whether you are exporting 24-bit or 16-bit FLAC from Audacity?