MongoDB command `compact` is not working

Question

I have a collection of documents in which I store an array of numbers. This collection is replenished with new documents about every 10 seconds. Every day I run a script which handles this arrays and leaves in documents only one number. After 3 month of data collecting I want to release disk space. Here is stat info of the collection from Robomongo:

{
    "ns" : "orderbook",
    "count" : 5363208,
    "size" : 844457120,
    "avgObjSize" : 157,
    "storageSize" : 2795139072.0,
    "capped" : false,
    "wiredTiger" : {
        "metadata" : {
            "formatVersion" : 1
        }
    }
    "nindexes" : 2,
    "totalIndexSize" : 144482304,
    "indexSizes" : {
        "_id_" : 90615808,
        "date_1" : 53866496
    }
}

As you can see the storageSize is much larger than the size. I expect that command compact will release disc space.

What I do in mongo shell:

use trading
db.runCommand({compact:'orderbook', force: true})

The answer is

{ "ok" : 1 }

Logs from /var/log/mongodb/mongod.log:

2016-12-26T11:19:55.468+0300 I COMMAND [conn7] compact trading.orderbook begin, options: paddingMode: NONE validateDocuments: 1

2016-12-26T11:19:55.562+0300 I COMMAND [conn7] compact trading.orderbook end

Nothing happens. storageSize has the same value.

Any ideas? What am I doing wrong?