File Size Discrepancies When Downloading iCloud Photo Library via privacy.apple.com

What might explain the following file size discrepancies when attempting to download the 5 "iCloud Photos Part x of 5" ZIP files that Apple provided, in response to my request for a download of all my photos?:

• I requested download files sized "up to 25 GB"
• privacy.apple.com's "Download your data" screen shows the first file ("iCloud Photos Part 1 of 5") has having a size of 39.02GB!!!
• Safari browser reports that the file it has downloaded is 47.2GB!!!
• Finder reports that the ZIP file downloaded to my disk is actually 35GB!!!
• Without prompting, Safari automatically uncompresses the Zip file, revealing contents sized as 12.17GB, when uncompressed!!!

Why this matters: I can't really be certain that I've obtained every photo file in my library, as 12.17GB in actual content bears no resemblance to the 39.02GB ZIP file I was invited to download. Further: privacy.icloud.com reports that 3 of my 5 downloads are 50+GB in size. As the website periodically pings me with "are you there?" -- requiring me to babysit the huge download, or otherwise risk a time-out, after which I'll have to restart the download from scratch, I'm wondering if it's worth the effort to sit and watch 50+GB downloads, when I may not even receive the full ZIP payload?