Why is my photo library 3 times bigger on my phone than my MacBook Air??

Then I would recommend a hard cut and reset your phone to factory settings and clean it.

Before you do this you must write down the names of all apps that are still on your iPhone, that will make it easier to install them later.

Check that all data / photos & videos are synced with iCloud

Check that you have all passwords / that keychain is synced with iCloud

Check if you have apps for two factor authentication on your iPhone and that you have the information to install them agin.

Logout from your iCloud account before you reset the iPhone

Erase iPhone - Apple SupportOfficial Apple Support › guide › iphone › ios

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Are you using any backup software (Time Machine) on your Mac?

Please remember that a backup will only save the small version of your photos!

It would be good if you have the full-size version of your library on the Mac, then it will be saved with a backup.

Ralf

Please check the size of your photo library in the 3 positions, iCloud, iPhone and MacBook.

And also the free space on iPhone and MacBook.

Optimised storage will only reduce the library size if there in not sufficient space on the device for the full sized library.

Ralf

I understand that part.

How many GB is free on your iPhone and how many GB is free on your MacBook?

I am asking for the available storage.

Example:

Your iPhone has a 23GB library but has still 20GB free memory available.

Your MacBook has a 8GB library but only 10GB free memory available.

The size of the library will be reduced so that it will fit on your device, if you have more space available it will be bigger.