I usually have my photos set to upload at "Standard Size" (2048x2048) so as to not chip away at my Google Storage. However now that I have my Google Photos in Google Drive (and vice versa) my photos are being AutoBackup(ed) at their full size.
I will add to this though that once I disconnect, and then disable and re-enable AutoBackup uploads are finally back to their "Standard Size."
Thoughts are always appreciated, but I think this may be something on Google's servers/app that needs to be taken care of.
Something that seems to have worked for a few people is this.
Go to your device settings
go to Applications
Go to All
Find the Google+ App
uninstall all updates for the google+ app
clear app data (this doesn't delete any photos, it just resets your settings)
Go to the play store
go to my apps
update your apps from the play store
go to your device settings
scroll to account
tap into google
tap into your email address
ensure all boxes are checked, so that sync is on for that item
especially the Google+, Google+ Photos, and Google+ Uploads
(Google Photos can be left off as it syncs your Picasa images to your gallery app)
Ensure sync is on for the whole device
Go to your device Settings
Go to Data Usage(this has moved to Accounts in Android v5+)
Tap the menu button
Check Auto-sync data
(depending on your device you may have a quick switch in your notifications drop down panel)
it should work as per normal now.
go to google+ app
log into app
ensure settings are to your requirements
go to the Photos app
go to the left side menu
go to On Device
if the Autobackup Cloud icon is highlighted blue tap it to switch it off for a particular folder that you don't want to be backed up.
This is optional
go back to the Google+ app
From the settings go to the autobackup settings
select the option to backup all photos now
do not dismiss the notification for it.
Do all the steps described you don't have to do the optional one, but please ensure that you do the steps in that order