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

[Bouke / edit 'B]

On 14 Mar 2023, at 13:07, Phil Rhodes via ffmpeg-user <ffmpeg-user at ffmpeg.org> wrote:
> 
> Also, it depends what you mean by 50% of the resolution. If your original video is 100x100 pixels and you reduce it to 50x50, then you have half the resolution, but you only have one quarter the pixels.

But, the image will still have a tree, a house and a dog.
From what I have understood, the resolution in terms of pixels does not compute to output file size (Unless you work uncompressed of course.)

> P
>    On Tuesday, 14 March 2023 at 09:37:36 GMT, Natalia Molinero Mingorance <eng.nataliamolinero at gmail.com <mailto:eng.nataliamolinero at gmail.com>> wrote:  
> 
> I'm using this command:
> 
> -y -r -i $inVideoUri -movflags faststart -c:v libx265 -s
> $videoResolution -c:a copy -preset ultrafast  $outPutUri

/// 
> some headers or metadata added during the process but I would like to know
> how to estimate the final video size.

Why is the output size important if you use -preset ultrafast?

Bouke