Why is my Photos library on my MacBook three times as big as my library on iCloud?

Are you by any chance using My Photo Stream in addition iCloud Photos? Or are you importing items manually from on iPhone? Combining iCloud Photos with other syncing services can create duplicates. Also running duplicate cleaners like "Clean My Mac" can corrupt the library so badly, that you will be left with dangling duplicates. have you been using any third-party application to remove duplicates?

If your library in iCloud is complete, and without duplicates, you could replace your existing library on your Mac by a fresh download from iCloud to a new, empty Photos Library.

Or do you have imported photos or videos to your library on your Mac, that are not yet in iCloud Photos?

It probably isn't. The library isn't a single file, it is a so called "package" (a hidden folder struture) and the top level size isn't updated properly.

To find out the true size, open the package (CTRL (0r right) click, and show package contents). DO NOT change anything in here if you don't want to irreversibly damage your library.

Check and add up the size of each of the subfolders. In reality you only need to size the "originals" (or masters, depending on OS Version) and "resources". The rest of the folders take up very little space.

I hope the duplicates have not been syncing back to iCloud, and you will get a clean copy this way. Good Luck!.