iCloud backup with photos

This might be a little confusing, but here goes nothing! I have my photos being backed up to iCloud, and that takes up a little over 100GB of iCloud storage (I know it’s a lot!) but when I look on my iPhone usage, it shows that photos take up about 15GB. My question is, when I switch to a new phone and restore from iCloud backup, will I lose the 15 GB of photos? Or are those 15GB also taking up some of the iCloud space? I’m sorry if that doesn’t make sense!