Why did my old iCloud backup get huge?

This is an odd one... backstory.

Back in Early November 2020 on my old iPhone 8+ I enabled iCloud photo and iCloud backup. All was good. Backup size was 7.3GB and photos was ~66GB. I did have save all photos locally to my phone as well as sync down to an iMac I have).

A few weeks later, near the end of Nov 2020 I upgraded to a new iPhone 12pro max. I put my old iPhone 8 in airplane mode and powered it down after the final backup.

I used the iCloud backup to restore it to the new phone. All went well. Photos re-synced etc. and in iCloud I now had a second backup created for my new phone leaving the old one there. Size ~7.3GB

About 2 weeks later, I removed the old phone from iCloud by signing it out and wiping it. Powered it up, connected to WiFi and signed out. I do not recall the size of the backup as this time.

Sometime near the end of December. iCloud photo in optimized mode got enabled on my iPad, I didnt want this, so I shut it off. I told it to remove the photos off this device, icloud photos/iphone photos were still good.

Fast forward to today, I went to delete my old iPhone 8 iCloud backup and its now over 70GB in size.

Date is showing last backup date of the date I cut over to my new phone.

New iPhone 12 is still showing 7.6GB backup size and current.

iCloud Photos still syncing between my iPhone and iMac ok ~68GB

What the heck happened to that old backup? and can I delete it safely?