When the image at the source is larger (higher resolution) than the image on Danbooru.
This may be because the artist revised the original image and replaced it with a larger version after it was uploaded to Danbooru, or because the uploader uploaded an Image Sample or Downscaled version of the image, or because they uploaded a smaller version of the image that actually came from a different source.
See also
- Source Smaller - the opposite case
- Resolution Mismatch