I’m puzzled as to why you can’t produce a 600 x 400 animated .gif of small size.
A .gif has the problem of a 256 colour palette limit so there’s the gradient banding and ‘speckle’ problems with using it but animated .gifs can be quite small in size and display well in a browser.
I ‘ripped’ your video from Vimeo to get a 1920 x 1080 .mp4 file then brought it into krita with the Animator Video Reference plugin:
That gave me a 1920 x 1080 animated layer at 24 fps with 144 frames using 1.1 GB of RAM and it saved to a 92.5 MB .kra file.
I did Image → Scale Image To New Size to reduce it to 610 x 430 and that Rendered out as a 3.4 MB animated .gif.
Looking at this in more detail, there’s a problem with rendering an animated .gif if you try to resize it with the Width: and Height: values.
Those setting have no effect but they do work for .mp4 output.
So, you’d have to scale it down (which takes a bit of time) before rendering as animated .gif and then Undo the scaling operation, or Quit and not Save.
I’ll raise a formal bug report for this.