We’ve released NSD 4.14.1 with more compact data storage for improved memory efficiency.
Check the release notes: https://www.nlnetlabs.nl/projects/nsd/download/

Willem also wrote a blog post explaining the approach and measurements showing the reduced memory footprint:
https://blog.nlnetlabs.nl/smaller-faster-nsds-refactored-rdata-storage-and-compile-time-memory-reduction-options/