Also, just an unlikely thought. Make sure your monitor has square pixels.
Hi @Per,
Not so unlikely. The panel used in that monitor has an active area whose height to width ratio (341.28/620.928=0.5496) is slightly less than the 4K ratio (2160/3840=0.5625). In other words the pixels are not quite square. The difference accounts for the 196/200 measured ratio of your circle.
By way of comparison, a randomly picked 32" IPS panel (from Samsung) has an active area ratio of 392.04/696.96=0.5625, giving a square pixel.
HTH
Jeremy
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One way to check this would be to make a square in Rhino, do a screen capture to clipboard (or file), paste into Photoshop and count the pixels in both axes there to see if they are the same.
Or even directly in Photoshop, use the rectangle tool, set the stroke width to 1 pixel, make a small square with Shift held down then measure the square on the screen. If it shows the same H/V scale difference as you are seeing in Rhino, then yes, your monitor pixels are not square.
Wow, Jeremy. Where did you get those details on the various monitors?
Also: I suspect that the issue is something the average gamer wouldn’t notice, what with all the action going on.
If you google the monitor, various sites give the panel detail, for example 28" Acer XV282K KV - Specifications (displayspecifications.com).
You can then google the panel model and you will find, for example Innolux M280DCA-E7B Overview - Panelook.com.
I agree about the average gamer not noticing (unless it’s a darts game maybe? 
) and Acer market this as a monitor for gamers, not creatives. But that said, there is nothing to indicate to a creative that there could be this very slight one-directional scaling. And I wonder how many other monitors out there have pixels that are marginally off square.
Regards
jeremy
Thanks. Nice research. Now that this has been brought to light people here will become aware of it as another thing to consider when purchasing a monitor.
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@jeremy5 thanks for your detalied info. And its not nice to realize that the screen you just bought dont have square pixels, which would be the obvious setup.
And Mitch, thanks for the idea to troubleshoot!
Sorry to say that Mitch recipe for checking the pixel “squareness” was correct! Its not… damn!!
Thanks all for pitching in!
Ouch… 
Guess I was lucky to get one with square pixels. Never really thought about this before, just assumed they were always square…
Have sent an email to Acer support asking if thats the case with this monitor but I asume thats it! Will post back if I get an answer!
Den tors 2 feb. 2023 12:48Helvetosaur via McNeel Forum <notifications@mcneel.discoursemail.com> skrev:
This is likely due to non-square pixels in 4K displays, causing circles to appear slightly compressed. Some graphics card drivers offer options to correct for this, otherwise you could try using the display’s built-in scaling options. If neither of these options correct the issue, you may need to use an external scaling device.
You must use “Zoom1To1Calibrate” command to calibrate your monitor.
Where can you find that setting? The only calibration I can do on the screen, is to lower the value of pixels on either height or width but it jumps in increments of 40 or 50 pixels at a time. Not only one pixel at a time.
I got a really fast answer from the Acer support, but they asked for the snid number of the screen and they still havent come back with any info about rectangular pixels
I’m afraid that command has nothing to do with height to width proportions - it is intended to calibrate the zoom one to one command so that objects appear actual size when so zoomed.
Regards
Jeremy
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No, launch the command and measure with a rule the blue rectangle and indicate the measurement in the box, it is different from zoom then 1TO1, try…
Indeed it is. It calibrates the zoom 1 to1. See the help text Rhinoceros Help - Zoom | Rhino 3-D modeling (mcneel.com).
As it only takes a single measurement there is unfortunately no way it can adjust the height to width proportions which is the nub of the OP’s issue.
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Vinci! I am sorry but Jeremy is correct its not a solution to my issue. Still waiting for a response from Acer…
If all your applications display an odd aspect ratio, and your screen is an unusual size, as 28" is for “4k”, then obviously no it doesn’t have square pixels.
@JimCarruthers what would be a “normal” size for a 4k screen? If I choose a larger 4k screen then the pixel size would be larger (am I correct?) and the visual appearance would be like a smaller screen with lower resolution. Dont know if my assumption is correct…