Desktop picture doesn't fill screen

You haven't mentioned the model year of your newer machine, but if you navigate to the original file that is being used, I suspect if you Get Info you will find that the resolution is less than what is now default on your newer iMac monitor. Since the graphics processors are not at full speed until you login, the basic drivers are doing the best they can with the lower res desktop image.

For grins, you might try a safe boot and see if maybe there's something that is confusing the OS.  Restart the iMac and hold the shift key down until you see the Apple logo and progress bar. It can take much longer (10 min) to safe boot so be patient.

How to use safe mode on your Mac - Apple Support 

Safe mode forces the OS to clear certain caches and logs and do other miscellaneous housekeeping. It's non-destructive to user data so is safe to do. Very often certain weird software behavior is corrected by simply logging in this way as a troubleshooting method. When you're in safe boot, the machine will not be at its best performance, especially with graphics, but that's normal.

Does the problem persist while in Safe mode?

Exit safe mode by restarting your Mac normally and re-evaluate the issue again.

seadub64 wrote:

Are there any instructions as to how to "scale it up"?

Preview>Tools>Adjust Size —make changes to the original and save.

Replace the existing Desktop photo with your "new" one.

example only:

Is this an image that is included with the macOS install or an image you have chosen from your own to use as a desktop image.

If it's your own, then perhaps the image is of lower resolution and therefore might be seen at its native size before you log in and the preferences setting forces it to be scaled or stretched to fit the screen. This is just a guess.

If it's an OS supplied image... I have no idea why that might be happening. 🙂

The image is smaller than your screen, and is being scaled after you log in.

Since you like the image so much and don't seem to mind or do not notice that it is scaled, you can open the image in Preview and scale it up.

seadub64 wrote:

So I would scale it to my iMac's native resolution of 4096x2304?

< 4096x2304 native resolution, >

Well from everything I have read in this thread above, that would be my conclusion if that is what you want.

No harm in trying is there(?)

First, make a copy of the file for safekeeping.

Once you do that, there is no harm in trying.

The smaller Retina 4K iMac models from the "Late 2015," "Mid-2017" and "2019" lines have a 4096x2304 native resolution, but by default run "pixel doubled" at 2048x1152.

+1 👍🏽