I think I know what might be happening here, causing some (or perhaps a lot) confusion.
Like @platypusman correctly stated (once again), the viewport setting is of course crucial in defining the initial view in the Preview. If the width of the artboard and/or the height of the viewport isn't larger than your monitor's (perceived) resolution, then the preview will be okay, unscaled, matching your 100% design. This is what's happening normally. Some users might be wishing for other initial behaviors or an option to change the behavior while viewing, which is not too crazy or wouldn't be too difficult to develop. But with only 58 users voting for this UserVoice request in 5 years, I think the current incarnation of the Preview isn't that bad.
But one of the UserVoice comments (Rishi's) made me go "ahhh..." as I read his tip, back in 2019:
2. Add Vertical Scrolling to your artboard.
That's it ! For all preset artboards the default Scrolling setting is Vertical. (Custom artboards don't.) As I switch it to None, the Preview immediately and correctly shows the whole artboard in the (live) Preview. There's your "Fit" Preview option or even preference, in a make-shift way though. But as I switch Scrolling back to Vertical again, I notice that the Viewport height is left at the momentary height of the artboard. It doesn't change back to any previous or preset setting. So if the artboard has ended up just a bit too high for your screen, the design keeps scaling to fit the Preview window, with black borders and all...
So I guess that might be a behavior which is causing some unexpected and conflicting results.
Fortunately, it's easy to tackle. Just set the Viewport height back to an obvious or feasable height.
The only use cases in which scaling options in the Preview would be useful, is when you're designing with exact pixel precision in mind for double or Retina resolutions (like one of the voters does), or when you want to accommodate your screen to –how appropriate– a Zoom session (like another voter does). But then again, I could come up with a dozen more challenging circumstances (and solutions) for Adobe XD to anticipate.
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