Thanks for clarifying. Although, my iPad also has space - over 20gb remaining, so why is it only using 7gb?
I'm not worried about device space, I have over 100gb remaining on my phone. It's more that I don't want to be perpetually upgrading iCloud storage just for photos. Although I've solved that now, I've exported albums to keep offline on my Mac.
Still, it's confusing me that my iCloud storage hasn't reduced as a result - it's freed up 15gb from my Mac, shouldn't it be similarly reflected in iCloud because deleting from Photos on one device removes them from the cloud?