How to get the size of single document in Mongodb?

I encountered a strange behavior of mongo and I would like to clarify it a bit...
My request is simple as that: I would like to get a size of single document in collection. I found two possible solutions:

• Object.bsonsize - some javascript method that should return a size in bytes
• db.collection.stats() - where there is a line 'avgObjSize' that produce some "aggregated"(average) size view on the data. It simply represents average size of single document.

When I create test collection with only one document, both functions returns different values. How is it possible?
Does it exist some other method to get a size of a mongo document?

Here, I provide some code I perform testing on:

1. I created new database 'test' and input simple document with only one attribute: type:"auto"

   db.test.insert({type:"auto"})
   

2. output from stats() function call: db.test.stats():

   { 
     "ns" : "test.test",
     "count" : 1,
     "size" : 40,
     "avgObjSize" : 40,
     "storageSize" : 4096,
     "numExtents" : 1,
     "nindexes" : 1,
     "lastExtentSize" : 4096,
     "paddingFactor" : 1,
     "systemFlags" : 1,
     "userFlags" : 0,
     "totalIndexSize" : 8176,
     "indexSizes" : {
           "_id_" : 8176
   },
   "ok" : 1
   

   }

3. output from bsonsize function call: Object.bsonsize(db.test.find({test:"auto"}))

   481
   

In the previous call of Object.bsonsize(), Mongodb returned the size of the cursor, rather than the document.

Correct way is to use this command:

Object.bsonsize(db.test.findOne())

With findOne(), you can define your query for a specific document:

Object.bsonsize(db.test.findOne({type:"auto"}))

This will return the correct size (in bytes) of the particular document.