Why does my DNG thumbnail show a wider image than my imported DNG file in LR?

first, "Original" is a setting of the Aspect Ratio part of the crop panel. This merely changes the shape of the current crop so it is the same shape as the original image. So if your camera shoots 3:2 natively, this will further crop down the current boundary, to a 3:2 proportion. Or if your camera shoots 4:3 natively, it will further crop down whatever you currently show, to 4:3 proportion.

The option you are looking for within the crop panel, which removes all cropping, is "Reset".

OR this could have to do with how some cameras have a multi-aspect-capable sensor. By that I mean: the entire effective sensor area is engineered larger than, and to encompass all of, the selectable aspect ratios for output. Corners of this sensor may not deliver high quality picture information though, due to lying right at the limit of the designed lens image circle / perhaps being optically vignetted or even physically masked out.

So for camera JPG, whichever is the currently chosen image aspect ratio gets used with the surplus "thrown away". And for Raw, this chosen image aspect ratio gets stated in the metadata, to be implemented during Raw conversion. However it is possible the embedded preview and certainly an optical viewfinder image, could derive differently from the overall live sensor data.

In this scheme setting 16:9 would thus maximise use of the horizontal sensor coverage, while discarding some of its vertical coverage. And setting 4:3 (or still more, square 1:1) would make full use of the vertical coverage, but with some - significant for 1:1 - consequent expense to its horizontal coverage.

For Raw, the full sensor information is in my experience included and then a utility from Adobe called "DNG Recover Edges" can change the implementation of image aspect ratio during conversion. Now the entire sensor effective area - warts and all - will become visible, but only once the default aspect-ratio crop has been Reset within Lightroom Classic.

If the image has been cropped, then it's likely that the embedded DNG preview has not been updated. You can update the embedded preview using the 'Update DNG Preview & Metadata' command, whcih can be found under the Library > Metadata menu