The size of photos library with many originals

Photos is supporting a lossless workflow. The original image file is always saved, unchanged. In addition, Photos is creating auxiliary files, a preview and thumbnails for viewing the photo in Photos and in the Media browser.

When we edit an image, add adjustments, rotate it, etc, Photos will store the sequence of adjustments we applied and the original will remain the same, but the auxiliary files will be updated. The edited version of the adjusted image is living in limbo, it does not exist, until we export an edited version. Only on export or when sharing the image, the edited version of the image will be rendered, according the export preset we are using, the format, quality, and dimensions.

Editing an image will not increase the library size much, unless we are using an external editor. As Photos cannot reproduce the external edits it will store the external edits as a second quasi master file.The additional files created by external editors can be huge. For example, Luminar 4 created huge TIFF files as edited versions and I found sometimes that a 3MB JPEG had an 80MB edited version created by Luminar saved back to Photos.

The lossless workflow in brilliant. It ensures always the best possible edits, starting from the original and does not increase the library size much. We can always revert all adjustments individually, and when we add further adjustments, the editing artefacts do not accumulate, as the adjustments will always be applied starting from the high resolution original and not to intermediate versions,a s long as we only use Photos and not an external editor.

The working copies (thumbnails, previews, data base files) need some additional storage, but they are also needed if we do not edits the photos at all. They are essential to make photos very responsive and fast. In my library the working copies are increasing the size of the Photos library by rough 20 % of the size of the originals.

I would not sacrifice the originals after editing a photo. At least save them on an external drive. I noticed that my taste and my editing preferences have changed over the years, and are frequently editing my photos again years later. And it is a lot of work to retrieve the originals from the backups, if we have not saved them for easy access.

... it is my understanding that if a library had every photo modified in some way, the resulting  size of the photo library would be about 20%  larger than otherwise , regardless of the intensity of editing changes.

That's not quite right. The 20% that léonie spoke of has nothing to do with editing. The edits are just the steps you took, essentially text which takes up practically no space at all. When you look at an edited image, Photos recreates all of your changes on the fly using the list of steps. No pixels are saved.

The 20% extra is for the thumbnail images that make it possible to scroll through the whole library without it having to load thousands of full pictures, and for similar previews that save time and make a smoother flow. All of this will still be there even if you do no edits at all.

So if you crop your pictures with an external editor, you'll have two copies of the pictures. If you crop your pictures using Photos, there will be only one copy each, and (invisible) instructions on how each was cropped. It will just look like there are two copies. As léonie says, brilliant!

As you suggest, I always keep a copy of the originals on a separate drive just for backup.

You're welcome. That looks like a good plan. 😊