[FFmpeg-user] Quality Reduced when Burning Subtitles

[Veronica & Stephen McGuckin]

Hello,

I am trying to overlay pgs subtitles onto a 4K video. I have extracted 10 minute to make sure I have the correct ffmpeg coding. The input file is 5.6 gigabytes with a bit rate of 78.6 Mb/s. I am using the following ffmpeg commands.

ffmpeg -i c:\video\input.mkv -filter_complex "[0:v][0:s]overlay[v]" -c:v libx265 -pix_fmt yuv420p10le -profile:v high  -x265-params "-crf=10 -film -hdr10+=1 -preset slow" -map "[v]" -map 0:a:0 -c:a copy c:\video\output.mkv

When the conversion is complete the output file is 143 mb with a bit rate of 279 b/s. I am surprised the output file is so small. I would have expected it to be also around 5 Gb. I am concerned that I will have lost some of the quality. Have I somehow got the wrong parameters? 

Any help will be appreciated.

Stephen McGuckin