@matigo It was in by default because I used the iMazing app on the Mac to set up the new phone as a restoration from a complete backup of the older phone.
Because the older phone already had all the photos on it & 15 GB per month is plenty of data, I set that option to on, reasoning that there’d be minimal data usage in maintaining the iCloud Photo Library. The new phone was exchanging lower quality pics for full-sized ones.