CLONE - How to use compression while inserting record?

I am developing application with PHP and Mongodb.
Recently I upgraded to v3.0 to use compression feature.
For configuration I run below command.
mongod --storageEngine wiredTiger --dbpath /var/lib/mongo --wiredTigerJournalCompressor zlib --wiredTigerCollectionBlockCompressor zlib

Now I am inserting record record in document but its taking same space in disk as it was taking without using compression tool.

if I run "gzip datafile" command, it compresses file.
But I was expecting like this zip process should be automated, while inserting record.

Is this possible to achieve this?