My tech said he’s coming across this complaint more frequently every time there’s a new iPhone out.
We phoned Apple helpline who advised I turn off iCloud backup/ delete all photos from new iPhone then reinstall from iCloud to see if all my photos re-installed from iCloud with lower storage BUT I WAS NOT WILLING TO RISK LOSING ALL MY PHOTOS!
You see if you delete some photos from iPhone, they immediately auto-delete from iCloud.
But the next time I upgrade iPhones (which may be sooner than I hoped) I will wait until I hear this photo storage doubling issue has been sorted by Apple.
Apple helpline also asked if I still had my old iPhone XS which unfortunately I’d reset to factory settings and gave away to family member (before I realised the photo storage issue).
This was because the auto-transfer from XS to 13 was so easy.
I await for an explanation or solution with interest.