Uploading image file mysteriously creates thumbnail in thumb folder

Trying to figure out where these thumbnail images are coming from. When I upload a new image, I noticed that in my thumbs folder, there appears two new thumbnails. One is the 75x75 one, which I assume is the one used in the panel as the preview image. The other is 400x266, which is weird because it’s a cropped and resized version of the original.

I’ve checked everywhere I can think of—config.php, various plugin files, all of the files I’ve created to see if I can figure out what is causing the extra 400x266 thumbnail but can’t find a single place in any of the files that could be causing it. At least, there isn’t anywhere that the thumb() or resize() functions are being used.

Wonder if anyone has any ideas?

That second one is also created by the Panel for the files view: