The ability to clean up space in Google Photos is perhaps the best feature of the whole program. Google announced the improvement in a Google+ post.
In my case, I was able to recover 8.6 GB of Photos storage space. The process involves converting “Original” sized photos to Google’s “High Quality” file format. I’m sure, at least for extreme photophiles, there’s some “quality” lost in the original versus high quality conversion. But for me, it’s too subtle to notice.
Also, the conversion doesn’t effect files stored directly in Google Drive. So if you do want to retain the “original” format for some pictures, simply use Drive to store those photos.