After restore, Photos is uploading items
I am having the same issue because I restored from an iTunes backup which had all my pictures inside (I bought the 256GB to have all my photos on the device). 
Doing so first created a 40GB extra iCloud backup for my iPhone from nowhere then added about 15GB of data to my iCloud Drive folder. No idea what is in there.
Have been in touch with two Apple advisors in these days:


the first told me that this is normal
the second one made me perform a turn off / on of the iCloud Photo library (which didn't help), then a log out/in of the iCloud account (which didn't help).


They said to wait 72hrs! I wonder how this is considered acceptable! 
I hope those two steps they made me do can help you.