I seem to remember answering a similar question in 2012.
The problem was that the download was straight from the camera.
Your screenshot seems to indicate that you too are doing the same.
It is never a good idea to download straight from the camera, for a lot of reasons.
Your issue is a minor one in the bigger scheme of things but still highly annoying if your workflow uses the loupe mode in the import dialog.
The solution is either to use a dedicated card reader or to download images onto the hard drive first and then only import into Lightroom.
My earnest suggestion is to get a good card reader!
If you don't have one right now, then, as an experiment, take a shot of your big toe or your cat or something and use the OS to download the image onto your hard drive and only then import it into Lightroom - you should get full size previews in the import dialog loupe view. That will prove that the issue is trying to download straight from the camera into Lightroom.
This whole issue has been grumbling on for some years now with no resolution with some of the blame needing to be directed at Adobe but not all.
And, I nearly missed it - welcome to Lightroom Forums!
Tony Jay