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.
This tag implicates resolution_mismatch (learn more).
The following tags implicate this tag: has_downscaled_revision (learn more).