I'm new to this compression thing. I use following command with the latest ffmpeg version to compress and downscale my 1080p videos.
ffmpeg -i input.mkv -af "aresample=async=1:first_pts=0" -c:v libx264 -crf 18 -preset medium -b:v 3000k -maxrate 3000k -bufsize 10000k -acodec aac -c:s copy -map 0 -vf scale=1280:-2 output.mkv
what i don't understand, two videos having same duration, only one audio stream. after compression one has filesize 2GB while other 5GB. Why so much difference?