[SOLVED]: I was experiencing the very same issue, whereas my new iPhone 12 Pro Max- 128gb had no space after restoring from my iCloud back up. My previous phone is a iPhone XS Max- 256gb, and I have 2TB of iCloud storage but my new iPhone 12 Pro Max didn’t have enough space while transferring data. You need to Download and Install iOS 14.2.1 (latest iOS) in order to fix this bug issue. Here’s what to do:
1) On previous iPhone- back up phone via iCloud
2)(on new device) Reset iPhone 12 Pro: settings>general>reset> erase all content and settings. Confirm by entering passcode.
3) Once iphone restarts- go through setup process again for iPhone 12 Pro, but this time setup as NEW PHONE. Do not restore from a backup (you will do this later in the steps).
4) Breeze through the setup process and skip through Face ID setup, skip Apple Pay setup, skip Apple ID login and click “set up later”.
5) Once you are in the phone and have bypassed the setup process- Go to Settings>General> Software Update> Update iPhone 12 Pro Max to iOS 14.2.1 (latest iOS)
6) Install iOS 14.2.1. After installing the new iOS software- Reset device again.
7) settings>general>reset> erase all content and settings. Confirm by entering passcode.
8) Once iphone restarts- go through setup process again for iPhone 12 Pro, this time restore from iCloud backup and previous iphone.
9) Complete the setup process by actually completing everything, including Face ID, Apple Pay, and other settings.
10) After your iphone finishes restoring from iCloud, you will have available storage!! Go to settings to check under General> iPhone Storage. You will see that you’re photos and other large apps are now accurate and the bug issue is fixed.
This worked for me, and hopefully it helps you too!