It sounds like it might be a bandwidth issue. If you waited around long enough in the import dialog, the information would transfer from the camera to the computer to build larger previews. Try importing while the camera only has a couple images on it and see if things work then.
I just formatted my card... took one picture and waited after i hooked it up to the mac and loaded up the import viewer on Lightroom.... after 10 minutes it was still the same... i did notice though, that when i had my iPad hooked up it was the same thing. This is making me think that maybe there is a setting option that minimizes the file quality before import from any external device...
The previews that you see in the Import grid before importing are the camera-embedded previews the camera puts in into each raw file, itself, OR that are rendered by some sort of intervening device-driver that is filtering what the camera shows the computer before the files are actually transferred, the same as what you’re saying at the end of your reply.
Does it work any differently if you use a card reader or plug directly into the camera, whichever of those you’re not doing now.
Can you see the camera or card listed under the Files section below the Device section? Are you using LR as it pops up when you insert the card or plug-in the camera? If so does it work differently if you initiate an Import operation from within LR, itself, after cancelling out of any auto-started Import?