Model was uploaded from a wide screen (highlight to see shape)
In the above example, "Kelso Town Hall" by Alistair_M, we can see the model was uploaded from a wide screen.
If you can't see it then highlight the image.
Model as it appears in a standard 4:3 window with shadow settings turned on.
Since the allowed space on the model preview page is always about 4:3 ratio, if you upload an image with a widescreen ratio, the result is that your model will appear very short (like a "letterbox" format with the top and bottom areas unused). Resizing the SketchUp window to about a 4:3 ratio yields a preview that will fill up the space as in the example above.
I usually like to turn on shadows too to give the model a brighter appearance. Check out the "My Favorite Thumbnails" collection for more upload tips.
EDIT: Upon further examination, it seems to me that the preview window is more of a 5:3 ratio.