MongoDB stores a String as a BSON Symbol with a fixed overhead of 39 bytes.

bson-utf8-overhead

var bson = require('bson').BSONPure;
var str = 'привет';
var buf = new Buffer(str);

console.log({
	str: str, // 'привет'
	buf: buf, // <Buffer d0 bf d1 80 d0 b8 d0 b2 d0 b5 d1 82>
	strLen: str.length, // 6
	bufLen: buf.length, // 12
	bufBytes: Buffer.byteLength(str,'utf8'), // 12
	bsonStr: bson.BSON.calculateObjectSize(str), // 65
	bsonBuf: bson.BSON.calculateObjectSize(buf), // 8320
	bsonSymStr: bson.BSON.calculateObjectSize(bson.Symbol(str)) // 51, 51 - 39 = 12 (bingo!)
});

/* Findings:
 * BSON Symbol overhead is fixed at 39 bytes.
 * BSON Symbol length (minus the overhead) is the number of bytes in a string.
 * MongoDB String data type is a BSON Symbol, see https://github.com/mongodb/mongo/blob/v2.6.3/src/mongo/bson/bsontypes.h#L130-L132
 */