@the_rock I do have some idea 😉 (Snapshots, CPUSE, upgrades, Blink... are all developed in my team)
Snapshots are simply packing the current root partition in a file - if you take a snapshot not during an upgrade, we copy the file aside. if you do it as part of an upgrade (no need to do anything - we do it for you), the upgrade is performed in a new root partition, so the old one is automatically and effortlessly packed into a snapshot. so the basic size of a snapshot is the size of the entire root partition (including disk space that was never used). we can later compress the snapshot by reducing its size to include only the used parts of the original partition, but still it would be several GBs.