There is not an explanation, but make the most of it. Higher quality is nice. It'll probably give you stronger DOF as well. :)

It is not really higher quality that one gets, but instead it is the quality that one would expect to get when choosing the render settings.  If you don't make the viewport really small the hirez final render setting you set are ignored.  Hard to believe RL has not addressed this.

What hirez final render settings are you referring to? There is only supersampling, which works lot better with large viewport render.
DOF and blurring might have better quality with small viewport, but again you can tweak it (now that we have lot of settings to control it) to make it work better with large viewport. I did not play with it much. Just need to experiment.
Not to mention that small viewport greatly slows down the render process. I only use small viewport render on limited number of frames where I have problem with opacity artifacts (mostly with hair).
Other than that.. I hide taskbar, hit Ctrl+7, open render window, then detach it to make viewport cover the entire screen and then start render.

Here is an 8 seconds test video from my current project.
First 4 seconds rendered with fairly small viewport. Speed is 0.2 fps. Look how "zigzaggy" and flashy glowing edges are.
Next 4 seconds - same frames rendered with largest possible viewport. Speed is 1 fps. Not perfect but glowing edges are whole lot better as camera moves around.

One more tip. With long and GI "heavy" scene, turn the Auxiliary light on during render. It does not influence the final render, but it does speed up the initial "scene for suppressing shadow" thing (still do not know just that the hell does it do :) )

That's an interesting issue you have exposed here.
This definitely needs to be investigated thoroughly, and we need more precise info than just "small" or *large* viewports.
What size is your "hirez final render" ?

Is this a progressive issue ? Meaning does the quality change progressively depending on the viewport size ? Or is there a viewport size threshold from which the quality change abruptly ?

What if you render your final images at a size higher than your desktop resolution (for instance if your desktop is 2560x1440 then render at UHD). In this case does the quality still change depending on your viewport size ?

--
guy rabiller | GOETZIWOOD STUDIOS
"N.O.E." (Nations Of Earth) Sci-Fi TV Show, Showrunner.

IMO, the answer/explanation of the "phenomena" should have been given by RL a long time ago, since it was first discovered in iClone 6.
So that we do not have to waste time experimenting and guessing. But RL keeps quiet about this.

The fairly small window I was referring to: ~500x300 px. I never fully tested anything smaller than that. And the reason being - render time is growing with geometric progression.
Dragging the timeline up with the above project and making viewport ~150x85, sets sender time to 1 frame per 3 minutes.
I have no patience to even render 120 frames to see the quality outcome (which I am sure would be terrible). I just had to kill the project after 3 frames :)
Now for large viewport, I only rendered at 1920x1080, which matches my current screen resolution. And I never tested anything beyond that as I had no needs... up until now...

Thank you for suggesting the higher screen resolution! I just set it to 4k, got stronger glasses (because I could barely see anything) and rendered 2k with the same 120 frames.
Outcome: 1.5 fps (which I do not care that much about). But the quality is now nearly perfect. There almost no flickering or pixilation on glowing edges.
It's like an anti-aliasing was set to it's max all of the sudden. So I have uploaded a new test video adding a 2k render on a full 4k viewport.



I have to render the rest of the video with 4k viewport though to get a full picture of the outcome. As I am afraid I might get unexpected results with some other aspects.

Arf', your last sentence make me think I've been tricked again by english wordings.

Don't tel me you are using the word "viewport" to describe your final render resolution/image ?

--
guy rabiller | GOETZIWOOD STUDIOS
"N.O.E." (Nations Of Earth) Sci-Fi TV Show, Showrunner.

Ahh - I see the conundrum here!  Problem is in the wording. It's BACKWARD! I
t's RENDER SIZE DETERMINES 'VIEWPORT SIZE' - Set your render size, and your will see your 'viewport' or 'screen' will change to match the proportions you have chosen under 'Render Size' Does this answer your question?  ~V~

       Val RogueAnanda

No, no.. I am still going to render my video at 1920x1080 px... but stretched to full 3840x2160 computer screen resolution in viewport. That is where I get the best testing quality result.

To me the viewport is the embedded window in which you work your scene. I thought the issue was that the quality of the rendering depend on the original size of that viewport ^^ I hope not !
Then there is the rendering window showing the final image. Never use the word "viewport" to name the final image rendering windows! It is so confusing :)
But perhaps I still don't get it..

--
guy rabiller | GOETZIWOOD STUDIOS
"N.O.E." (Nations Of Earth) Sci-Fi TV Show, Showrunner.