I use the "original quality" option in Google Photos. Most of the photos were added via Google Drive folder (as files). Recent photos from phone added via Photos Android app (they also visible on Drive). I expected that storage will be used only by Drive, but see this in storage usage info:
Drive 74,14 GB
Gmail 0,41 GB
Photo 10,56 GB
Windows Explorer says that Drive folder is 80 GB.
What is counted under that 10 GB?
I have only one album in Photos with files less than 1 GB. Blogger/Picasa/etc – no more than 500 MB.
I found that "If you delete folders (including the Google Photos folder), photos and videos won’t be deleted in Google Photos." (from How Google Photos works with Google Drive) But can't imagine that I have deleted folders with 9 GB of photos on my drive.
Best Answer
You can consider Google Drive as your ultimate storage of all your data in your Google Account. By "all data" it means: pictures, emails, other data, so even if you don't have direct access to your emails from the Google Drive environment, your emails are still part of your storage. In a similar way, under "Photo" label can be understood pictures from Picasa/Photos/Blogger/Hangouts/Drive etc.