I just migrated from Mongo 2.6 (Single node) to 3.2 (ReplicaSet).
In the old server the Mongo's data folder was a total of ~12GB, but looks like in the new servers (master and its slave) it's just ~2.5GB - make sense? Has Mongo enhanced the way they're holding data?
The size I mention above, ~12GB includes indexes (that have significant size in my DB, almost double it).