A snapshot does not have an absolute size. It references blocks of data and metadata, which forms a complete filesystem.
The amount of space a snapshot “uses” is how many of these blocks are unique to the snapshot itself.
If you want a less technical way to approach it, check out this thread.
This is because the snapshot names match the pattern of a Periodic Snapshot Task or naming schema. You can specify a more granular selection in the Replication Task’s options, but it’s really not worth it.
See my first comment.
You can keep snapshots forever if you want. TrueNAS probably suggests pruning so that you don’t fill up your pool if your snapshots contain large amounts of unique data. It’s also nice to keep things tidy.
Supposedly TrueNAS tries to prevent this by not pruning common base snapshots from the destination before a successful incremental replication. Supposedly.