I cannot determine whether this is ultimately a browser issue or a Stack Exchange issue, I don't have the chops to delve that far. It does reproduce on every browser I have access to. It is not, however, an Imgur issue, nor an Apple/Retina issue.
It happens because the DPI is not being handled correctly.
For anyone not aware of how SE handles original uploaded images when inlined, it will reduce the apparent size of any image to a maximum display size of 650px wide. It will not enlarge any image.
This breaks perceptually if an uploaded image was saved at 144 DPI rather than the standard 72, but still the maximum displayed size on screen will be 650px.
I thought initially that Imgur may be stripping the DPI tags on upload, but this seems not to be the case. Imgur doesn't strip the DPI value, I have tested random uploads from other users [on Windows] where the DPI value survived uploading to Imgur, and have now confirmed this for myself from two different Macs.
Exiftool shows this information as x/y values, like this:-
X Resolution : 72
Y Resolution : 72
I don't know the provenance of the OP's images - The one from the referenced post says it comes from an HP device running Windows, with a CRT display, rather than anything from Apple. It contains no DPI data at all.
Link for future reference, as the image has been replaced in the current version of the question - https://i.stack.imgur.com/H4Wo3.jpg
The screenshot in the question above is from an Apple device, but the DPI data is rather odd:-
X Resolution : 1
Y Resolution : 1
I have checked old & new Mac screenshots on my own Machines. An old non-Retina Mac doesn't include DPI metadata in a screenshot, but as it's using the default 72 DPI anyway, this is not a problem. A new Retina iMac saves the non-standard 144 DPI in the metadata.
This survives upload to Imgur, but seems to be ignored by the browser, so appears double size, on both retina & non-retina Macs, also on a non-retina Windows VM.
Example retina screenshot you can test. It appears here double size, yet the DPI information is still present :-
When dragged back to my desktop off the page, this shows in exiftool as
X Resolution : 144
Y Resolution : 144
For comparison, the same image grabbed via remote desktop on an older non-retina Mac, at 72 DPI. No DPI values were saved in the original screenshot, but Imgur hasn't added any either.
I already proposed that this issue be looked at when we lose imgur next year - Stack Exchange’s image hosting arrangement with Imgur is ending - advance notice