That just gives you unlimited storage on Google's server, it will still eat through your data quota if you have enough photo that you upload and download. To avoid using your data, turn off "cellulare data backup", you'll still download the pictures when looking at them though.

I think I firgured it out, I have to change the quality preference on the photos I have and then it'll reduce back to no storage used, just don't have a suitable back up atm so I'm gonna do it another time.