A snapshot is actually just a struct with a few transaction IDs (xids) and some other bookkeeping that describes which slice of the physically stored data is su

A snapshot is actually just a struct with a few transaction IDs (xids) and some other bookkeeping that describes which slice of the physically stored data is supposed to be visible to a transaction. The article shows the details of that. So the size of a snapshot is unrelated to the size of the database.