Hmm, seems it won’t let me change the thumbnail size to 0 in Settings > Media. Just immediately reverts back to 10000. Is there another way to do this?
After messing around a little more I found that the image only becomes a 10000 x 10000 when I specify the aspect ratio as 1 to make the thumbnail a perfect square. It makes a normal size thumbnail otherwise.
Edit: Changed the value in functions.php and I’m able to make perfect square thumbnails now. Thanks for the help!