New phone, same gig, more $ for iCloud

iCoud Storage you pay for and iPhone storage are 2 different things unrelated to each other.

The fact that you have the same storage on your new iPhone as your last has nothing to do with how your iCloud storage is being used.

If you set the new device to backup to iCloud, then your iCloud storage now has a new backup to store there. So you are using double the storage in iCloud as before, since there's a backup for your new device and a backup for your old device.

One possible reason is that you still have a backup of the old phone stored on iCloud and have added a backup of the new phone.

Also see the following: Manage your iCloud storage - Apple Support