[FFmpeg-user] why reducing the resolution a percentage doesn't reduce the video the same proportion?

[Natalia Molinero Mingorance]

I'm using this command:

-y -r -i $inVideoUri -movflags faststart -c:v libx265 -s
$videoResolution -c:a copy -preset ultrafast  $outPutUri"


However, if "videoResolution" is a 50% of the original video resolution,
the resulting file size is not 50% of the original one. I assume there are
some headers or metadata added during the process but I would like to know
how to estimate the final video size.