Twerppoet, thanks for suggestion. Photos use 2.0 Gb. Photo Stream is 395kb. So that is not the difference.
The only significant PhotoShare file (268 photos of 632 total photos) I have is Family Shared, mostly from my photos. Surely those would not be full size duplicates but rather pointers to my All Photos.
I have put many of my photos into sub albums but again I should think these are pointers to the main photo not full size duplicates. Deleting from All Photos deletes the photo in the sub folder but not vice versa.
I cannot open Drop Box photos on the iPad when not connected to the cloud indicating it only stores thumbnails on the device.
Other thoughts?
Albums are pointers, Shared albums/photos are not. Consider the difference in how you make albums and shared folders, and how they have to work.
To move a folder to an album, all you have to do is select it, then choose Add Too; all wihtin the app, no Share/Action Sheet. To place a photo into a shared album you have to use the Action/Share sheet (the box icon with the up arrow). Whenever you use the Share Sheet you're almost always dealing with another app, or some separate feature of iOS.
And then there is the difference in how they are used. An album is just a collection in your photos app. It's only there for you and it can only contain photos that you add; an organization tool. A shared album has to be there for everyone you share it with, and if you give permisison other people can add, delete, or edit those photos. Moreover, if you are on the recieving end of a shared album the last thing you want is someone with the ability too add photos to your main Photo Library and use up your iCloud Storage space.
You can delete phtos form a shared album, or from the photo library, and they have no effect on each other.
That said, shared albums do tend to optimize. Older photos often take a bit to download and display at full resolution, just like photos in the iCloud Photo Library.
So, yes, shared albums do take up space on the iPad separate from the photo library. So does the PhotoStream. Combining the two probably makes up the majority of the difference you are seening; though I can't guarantee that.