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