Hi River,
thanks for the Detailed Post about your Setup, that's Great Savings on your DR Site! and Yes I can understand Space Usage can become very confusing especially when your DR Destination with more snapshots takes less space then your Production with less snapshots. In general only a DP Mirror will copy a exact block based Copy to the Destination and therefor its hard to compare a source with a XDP Destination as they are not Block related but the logical Data is related.
Please consider:
> you have 2 different keep patterns which can significantly alter data grow rate
-Source 4x 12h Snapshots + Dayly Snapmirror
-Destination: 30x 24h Snapshots
> your Destination is using on Top of Deduplication, Compression which can significantly boost up Space Savings depending on the Sour Data.
please let us know if you have more questions on this.
Detailed information's about Dedupe and Compression can be found in our Technical Report
TR-4476 NetApp Data Compression, Deduplication,and Data Compaction