Pictures' real quality?

[phaolo2]

Browser(Chrome/Firefox): 128.4.0esr
Extension Version: 2.2.4
Reproduction of links:
https://www.instagram.com/vivi_0.85/p/DCJ3KIIyu77/
vs
https://www.reddit.com/r/Frieren/comments/1gpjlub/%C3%BCbel_cosplay_by_vivi_085/#lightbox

I'm puzzled about the obtained pictures' quality. They like seem.. upscaled and with a worse quality than the same ones downloaded by other people. You can see more compression glitches and squares.
Is this extension working correctly to obtain the best quality?
(p.s: I noticed this also in other cases)

[Nissemus]

I don't see any difference between the images downloaded from IG and those downloaded from Reddit. Using Firefox 132.0.1.

[phaolo2]

Zoom a bit to see the difference. For example check these cropped parts


See the compression?

[TheKonka]

They do have slight differences, but this could also be due to the processing done by Instagram and Reddit platforms. I am not sure if the images I downloaded are the most original, but they are the best quality I can get at present.