960×540 is the size of the embedded jpeg preview in the dng. try some other software that loads the raw data and not just the preview image.
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Yes, I've just learned that it is the preview image. I used the default software on a PC for opening these. If I use Lightroom, it's OK.
Do you see that lower resolution in the file details, or in your raw converter? If the former, maybe it's only the metadata that got tangled up somehow, or Finder is reading it differently. Try opening the file in your raw converter.
Raw formats also usually save a smaller resolution preview image of the photo within the metadata, so that there's something to show about the unprocessed raw data. It's likely that whatever OP is using to view the photos are only dealing with the preview.
I opened the file in Lightroom, it worked well. Original size and resolution.