preview size on iphone icloud photos

Hey community!

I got 60000 photos in icloud photos with a size of 1tb. All fine.

My iPhone 7+ runs out of storage by using more than 6gb by photo app.

Optimize Storage is checked in photo settings.

The same happened to my iPad and iPhone SE.

How can i control the size of the previews? I dont want the photoapp to use that much space.

Thanks a lot!