LG G4 Bootloop--how do I get my pictures back?
Unfortunately, Google Drive does not backup your photos directly from your Gallery. It only backups data, like user settings and probably cache, from your various apps. Next time, if you want to backup your photos and videos, use Google Photos which would have eliminated this issue.

It seems that accessing your (and my) phone's backup is not possible through the web version of Google Drive. It is however possible, in the Android version, to see the size of the data being saved and by what app.