Google gives you a choice when you back up your images to Google Photos: do you save them at their original size or do you let Google store a compressed "high quality" version? The former counts against your storage space, while the latter doesn't. Unfortunately when you choose one, you're kind of stuck with the decision. You can opt to change how you save future photos, but you can't go back to compress those shots you previously saved at their original quality.
This is about to change. Tomorrow, Google will introduce the option to downgrade those previously uploaded images. This way you can free up storage space you no longer want taken up by photos. You will continue to have the option to store an unlimited number of high quality snapshots instead.
Once the feature rolls out, the setting will appear at photos.google.com/settings as Recover Storage.
While you wait, be sure to grab the latest version of Google Photos that's currently rolling out.