Compression in MongoDb 3.2 does not work
--version 3.4.10 Enterprise

compression will not change the avgObjSize. It will just improve the compression ratio saving disk space; I tested it for a collection (documents: 11553423 and avgObjSize= 6182) worth 21GB  with snappy, when same created with zlib, size of collection reduced to 12GB (avgObjSize= 6182).