Why doesn't WP Smush Pro optimise images uploaded via Nextgen Gallery plugin?

I've been wondering why my bandwidth limit was exceeded several times over the past few days. It appears that WP Smush pro doesn't optimise images uploaded via the Nextgen Gallery plugin. When you create galleries, images are uploaded to the wp-content/gallery folder. That folder is now 2.25GB in size! How can I optimise these images?