Media library loads “medium” size images, not thumbnails

I just noticed the image files in my media library are not the thumbnail size (default 150×150), but in fact the next size up: medium (default 300×300). Why?

It’s especially puzzling when you consider the thumbnail, by default, is cropped to be square, whereas the medium is not necessarily square, and yet the images are cropped using CSS on the library page. On that note, when resizing the window I actually see the rectangular shape of these medium-sized images (225×300, or 300×225) for a brief moment before they’re re-cropped with CSS.