what would be more massive would be phones not booting up because of a botched update. this way you can just switch back to the old partition

Not really, because only the OS core is swapped in this way. Apps and data live in their own partitions/subvolumes, which are mutable and shared between OS versions.

The OS core is deployed as a single unit and is a few GB in size, pretty small when internal storage is into the hundreds of GB.