[Plugin: NextGEN Gallery] medium sized file for medium sized image?

How do you resize the images so that the medium image is actually a medium-sized file?

It seems to only do thumbnails and full-sized images. The intermediate image may be displayed with a 500px width and a 375 px height, but it still requires a 3 MB download as the file itself is the full-sized image.

How do you resize the intermediate images?