Elastic 5 > 7: Decrease Storage Usage?

Hi!

I'm upgrading my ES cluster from V 5 to V7, I've noticed that space usage has significantly been reduced, is that normal?

my V5 cluster: 10B Documents, ~2.2 TB

V7 Cluster: 10B Documents, ~1.5 TB

(Same exact documents, a little tweaking to mappings since some settings were removed/added, no sorting was done)

Where can i read more into why/how this is happening?

There's been quite a large number of efficiency improvements between those versions. I am not sure there's a single place to look at as a result.