Twitter hides the original resolution. You grabbed the "easily available full size," while Cat-In-Flight grabbed the original full size, which is greater. You can find this yourself by suffixing the URL to the source with :orig.

The artist themselves might have a better copy. You can try sending them a note on Furaffinity and asking if the original is available, as they sadly didn't fix them there (They're still JPG (despite saying they're png) and are the default limit of 1280).