[FFmpeg-user] Truncated file with ffmpeg but resulting output is x2 bigger!?

[Moritz Barsnick]

On Fri, Oct 16, 2020 at 20:26:16 +1100, FFmpeg user discussions wrote:
> I truncated a 14s file with:
>
>    ffmpeg -i 20201016_055828.mp4 -t 00:00:09 -async 1 cut.mp4
>
> and although about 35% of the frames were removed, the resulting mp4 was
> double the size - why is this?

You are reencoding the streams inside the file. ffmpeg's default
encoding parameters apparently result in streams with more bandwidth.
You can fine-tune the encoding parameters, of course.

Re-encoding is not a good idea, as it always loses quality. If you just
want to "clip", be sure to use the "copy" codec:

$ ffmpeg -i 20201016_055828.mp4 -t 00:00:09 -c copy cut.mp4

(Note that cutting without re-encoding may or may not be exact. Feel
free to test first though.)

Cheers,
Moritz