As far as I can see when there are multiple posts of the same image but with different quality/resolution we keep the higher quality one. This makes sense.
Very rarely it happens that somebody flags the higher quality post for deletion for being of higher quality. This surprises me because there is almost no reason to keep a lower quality version instead. An example for an exception would be: The higher resolution post is just an upscaled version of the lower resolution one and does not actually have a higher image quality. In this case it makes sense to delete the upscaled post.
I can kinda understand why one could say that a smaller resolution would work fine with this post:
post #667333
As the artist explained, their style is kinda sketchy. So even though the bigger image actually had a higher quality (that is: it wasn't just upscaled) it was kinda OK to delete it.
But I really don't understand why this post was deleted:
760629
(this is the lower quality version of that:)
post #760631
It wasn't upscaled, it wasn't an absurdly high resolution, it was just higher quality. I also don't understand why the uploader would not want to upload the higher quality version.

Soo... why?

I am not trying to call anybody out. It's just that I don't understand the decisions made and I would like to be enlightened.

Because the uploader is the artist and if they only want to publish a downscaled version then that is their perfect right.

You can literally read the deletion reason stating that they didn't want to upload the large version, so it's just common courtesy that we remove the unwanted version.

NotMeNotYou said:

I did not recognize that the artist is the uploader because the person who uploaded and flagged it is "GloriousWings" and the artist tag on the post is "justice_(artist)", which looks very different. I suspected something because GloriousWings wrote:

[...] what my client told me.[...]

But I didn't quite make the connection there.

I do understand however that if the artist does not want to upload a certain image, it is absolutely OK to delete that post. (I'd still like to understand the reason though.)

Also I did not understand the reason in the flag description:

Uploaded HQ version by mistake.

Ok, that can happen, but it does not explain why the uploader does not want the high-quality version to be here.

Newest version is right size, as per the filename.

As the original file name is nowhere to be found I don't know what this means. Also this does not explain the reason.
And "Sorry" being in there made me think that the uploader thought it was not OK to upload high-quality images. But I guess it was meant like "sorry for bothering the mods".

I'd like to add here that a relatively high resolution is important for desktop wallpapers. 904 (which is the width of the lower-quality image) x 509 (on a 16:9 screen) would not be enough for a 1600 x 900 screen (or even 1366 x 768). The upscaling would be very noticable. Something like 1280 x 720 would be enough for that, but it would still be too small for 1920 x 1080.
An image like post #403746 is perfect because even on a 1920 x 1080 screen you can downscale it.
I am not saying that I want to use post #760629 (or #760631) as a desktop wallpaper, but I want to explain that desktop wallpapers are a thing.

High quality / full resolution files are generally kept between the artist and the commissioner, in this case the artist likely only wanted to share the full resolution with the commissioner, and thus had it removed.

As for the filename thingy, they stated they were tired, they probably exported 3 (or more) different versions for FA, the commissioner and maybe tumblr or something. And then picked the wrong file for e621.

Personally I don't know why some people only share lower resolution files, I enjoy uploading the highest resolution I receive from the artists.

We know that desktop wallpapers are a thing. If anything, IMO, e621 tends to err on the side of excessively large images (when they are available).

BTW, if you have a decent upscaler -- eg waifu2x or DCCI , upscaling isn't very noticeable at all, up to a factor of 200-300%.