What do you have your upload settings set to? Only on WiFi? Are you also backing up videos?
What's the quality they are set at? Google Photos allows two options for uploads. HQ and Original.
Also, is this the first time you've initialized Google Photos, because if that's the case then it likely uploaded every file you have for a 'first-run' upload and as such, subsequent upload usage wouldn't be as much.
09-12-16 12:50 PM