Quick Look usually scales down big images to suit the screen resolution. Because of this, we may lose some details of the image while we observed it in Quick Look mode.
Some people maybe suggest using Full Screen Mode, but that's not solving the problem for images with higher dimension than our screen resolution.
The ideal solution is to use Quick Look Zoom that is achieved by pressing (and holding down) Option key and you will see the cursor change to a magnifier icon. After the zooming, you will be able to drag the image around.
Zooming in effect:
 Normal View
 Zoom View
For zooming out, use Shift-Opt with mouse-click(s).
This trick only works on image files. If you want to zoom in/out PDFs, you can simply use Cmd-[+] or Cmd-[-].
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