I am sorry for the extremely late response. I know it’s been a long time but I only saw your question now (I didn’t get an email notification or it might went on my spam folder).

You probably don’t need it anymore but I will answer anyway. You can’t avoid keeping the originals and that’s intentional. Otherwise it would be rather dangerous (you might do some irreversible damage to your photos by choosing a very poor quality setting).

You might want to take a look at Imsanity (https://wordpress.org/plugins/imsanity/) which resizes the original images to a more sane size. It’s not exactly what you ask but it might help.