Is MongoDB producing more than 100% storage overhead? i.e., I insert 22GB data and it occupies 50GB on the disk

I have done an simple experiment to test MongoDB's performance and
disk usage. I insert 22GB data and it occupies 50GB on the disk. I
will describe this experiment in details as below.

Setup:

     Version - MongoDB 2.0.2
     Environment:
          Single node without any replication or sharding
          VM via VirtualBox
          Linux Ubuntu 64bit
          100GB fixed virtual disk and 1GB memory

Experiment program:

     Language: C# && MongoDB C# driver
     Target and Procedure: Very simple. I just constantly create a new
{KEY, VALUE} pair and insert it into MongoDB.
     Settings:
          Number of Insertion = 1024 * 1024 * 1024 / 3
          Size of the KEY = 20 bytes (byte array), a counter with
increment of 1 for each insertion, i.e., KEY = {1, 2, 3, ...,
1024*1024*1024}
          Size of the VALUE = 100 bytes (byte array), randomly
generated through Random class.

Results:

     So this experiment means I wished to insert about 40GB of data
(120 bytes of data for each insertion) into MongoDB and I believe it
is simple enough. However, I stopped when the actual inserted data
reached 22GB because I found the storage overhead issue. The actual
data I inserted is about 22GB, but all the indexdb.* files are with
size of 50GB. So there is more than 100% storage overhead.


My own thoughts:

     I have read quite a bit of MongoDB's docs. According to what I
have read, there might be two kinds of overhead for the storage.
     1) the oplog. But it is meant to be capped about 5% of disk
space. In my case, it is capped about 5GB.
     2) preallocated data file. I didn't change any settings of
mongod, so I think it is 2GB in advance. And let me assume that the
latest 2GB file in use is nearly empty, so totally at most 4GB
overhead.

So from my calculation, whatever size of data I insert, there should
be 9GB overhead at most. But now the overhead is 50GB - 22GB = 28GB.
And I don't get a clue what is inside that 28GB. And if this overhead
is always more than 100%, it is quite a lot.

Can any one explain it to me?