iCloud photos take up less space in the cloud than locally?

Question

So I decided to put my pretty big photo library on iCloud. I did this using the Photos app for macOS. After a few days, all of them seem to have successfully uploaded. There are about 70,000 pictures on my Mac and now I can see the same number on my other devices. However, when I took a look at how much space they take up, I found inconsistencies. Locally on my Mac, the photos library file is about 209 GB, whereas in iCloud, they only take up about 150 GB. Any ideas how that's possible? Did it upload all of them? Does iCloud use some sort of special compression algorithm? Thanks, Steve

Answer 1

The page at iCloud Photo Library says:

File types that you can use with iCloud Photo Library

Your photos and videos are stored in iCloud exactly as you took them. All of your images are held in their original formats at full resolution — HEIF, JPEG, RAW, PNG, GIF, TIFF, HEVC, and MP4, as well as special formats you capture with your iPhone, like slo-mo, time-lapse, 4K videos, and Live Photos.

However, what it doesn't talk about is the underlying storage used for those files, and the relative filesystem block sizes of their storage versus yours. It is entirely possible that they might be using a smaller and more efficient block size than you are.

Moreover, I strongly suspect that there are additional things being recorded in your local iPhoto Library that are not being uploaded -- like thumbnails.

The only way to know for sure would be to re-download some of the "originals" from the iCloud library and then compare those to your local original files. If that checks out, then at least the photos themselves are being stored as-is, even if there are other things in the library that aren't getting copied up because Apple doesn't think that it is necessary.