>>3386851That particular image isn't (as it's already under 1200 pixels in either dimension), but typically _master1200 images are resized. You have to expand it once more to get the original size.
>>338717223839775 is the illustration ID, you can get to the original like so:
https://www.pixiv.net/member_illust.php?mode=medium&illust_id=23839775 