Photos taking up space on iMac

Image files are large, depending on the resolution and the format of the image files. For example, my Photos Library has a size 250GB for 50000 images and videos. I am not using "Optimize Mac Storage", because I want all my photos and videos also local on my Mac, not only in iCloud. Otherwise Time Machine would not be able to back up my photos, and I could not work with my photos, when I am away from the internet.

You can save some storage by syncing your Photos Library with iCloud photos and turning on "Optimize Mac Storage". Then iCloud Photos will remove some of the large original image files from your Mac and keep them only in iCloud to save storage on your Mac. But you cannot remove the library completely from the Mac. You will still need at least 10% to 20% of the full size of the iCloud Photos library locally on your mac, for the thumbnails of the optimized versions, so you can browse the library, also for the database files with the albums, adjustments, folders, metadata. And Photos will only optimize your photos when the storage is getting low, not needlessly, if you still have plenty of free storage - this is fully automatic. Otherwise you would have to download the same photos over and over again to work with your photos. You can force Photos to optimize the library immediately, if you are running out of storage and need more storage urgently: How to force Photos for Mac to Optimise the Storage Immediately

But then do not forget to make a backup of your photos. It is a lot of work for an optimized library: How to back up an optimized iCloud Photos Library - Apple Community

One reason for a large increase of the library size can be external editors. When I edit a photo with an external editor, the edits are saved back as huge TIFF files, sometime more than 80 MB. But this new size is not shown in the Info, I can only see it in PowerPhotos, where the size of the edited version is shown.

The Library size can also increase considerably, if you are syncing photos from your Mac to an iPhone or iPad. Then Photos may create a photo cache folder, that can be huge.

Sometimes the size of the library is also shown incorrectly, because the library is a package of folders and files, and the Finder is not always updating the size of the package immediately. You may want to check, if the modification date of the Photos Library is current. If it is not current, the total size may be incorrect.

Or look inside the Photos Library, what is using the storage, but carefully, just have a look, don't accidentally modify something:

• Select the Photos Library, then hold down the CTRL-key while you click it. From the contextual menu use the command "Show package contents".
• In the Finder window that opens look at the sizes of the folders inside the library. The largest folder should be the folder "originals" with all the original image files, followed by the folder "resources" with the working copies, mainly thumbnails and previews of the edited versions. This folder has typically a size of up to 20 % of the size of the originals folder.

Do the total sizes add up to the size shown in the Finder? Are there other unexpectedly large folders?