That’s not how lossless compression works. No data is lost.
For example, if you zip a folder of images, then unzip them, the pictures come out with their original sizes and structure. Zip is lossless.
Let’s use the analogy of a dish sponge.
Let’s pretend you wanted to make a dish sponge smaller. Lossy compression would make the sponge smaller by cutting off parts and throwing them away. Lossless would make it smaller by squish the sponge, and it would return to its normal shape once you stopped squishing it.
I’d disregarded compression as a possibility because the wording is “full resolution photos and videos are safely stored in iCloud”
There is lossless compression. Not saying that’s the cause of the varied number, but it is a common thing.
They’re already using HEIC/HEIF
I would be disappointed if they’re compressing it even more on iCloud. You can’t generally meaningfully compress a compressed file
That’s not how lossless compression works. No data is lost.
For example, if you zip a folder of images, then unzip them, the pictures come out with their original sizes and structure. Zip is lossless.
Let’s use the analogy of a dish sponge.
Let’s pretend you wanted to make a dish sponge smaller. Lossy compression would make the sponge smaller by cutting off parts and throwing them away. Lossless would make it smaller by squish the sponge, and it would return to its normal shape once you stopped squishing it.
For your analogy, you can’t put more water in a sponge that is completely saturated
Trying to compress a compressed file doesn’t really work - at least not for a meaningful gain in storage size with zip, bzip, 7zip, gzip, xz, lzma…