why can`t i set server_options?

[leekafai]

//log
0|bh       | { emitter: { domain: null, _events: {}, _maxListeners: 100 },
0|bh       |   state: 2,
0|bh       |   _dbconn:
0|bh       |    { domain: null,
0|bh       |      _events: {},
0|bh       |      _maxListeners: 10,
0|bh       |      databaseName: 'XX',
0|bh       |      serverConfig:
0|bh       |       { domain: null,
0|bh       |         _events: {},
0|bh       |         _maxListeners: 10,
0|bh       |         _callBackStore: [Object],
0|bh       |         _commandsStore: [Object],
0|bh       |         auth: [Object],
0|bh       |         _dbStore: [Object],
0|bh       |         host: 'localhost',
0|bh       |         port: 27017,
0|bh       |         options: [Object],
0|bh       |         internalMaster: true,
0|bh       |         connected: true,
**0|bh       |         poolSize: 1,**
0|bh       |         disableDriverBSONSizeCheck: false,
0|bh       |         _used: true,
0|bh       |         replicasetInstance: null,
0|bh       |         emitOpen: false,
0|bh       |         ssl: false,
0|bh       |         sslValidate: false,
0|bh       |         sslCA: null,
0|bh       |         sslCert: undefined,
0|bh       |         sslKey: undefined,
0|bh       |         sslPass: undefined,
0|bh       |         name: 'localhost:27017',
0|bh       |         _readPreference: null,
0|bh       |         socketOptions: [Object],
0|bh       |         logger: [Object],
0|bh       |         eventHandlers: [Object],
0|bh       |         _serverState: 'connected',
0|bh       |         _state: [Object],
0|bh       |         recordQueryStats: false,
0|bh       |         db: [Circular],
0|bh       |         dbInstances: [Object],
0|bh       |         connectionPool: [Object],
0|bh       |         isMasterDoc: [Object] },
0|bh       |      options:
0|bh       |       { safe: true,
**0|bh       |         poolSize: 8,**
0|bh       |         strict: true,
0|bh       |         username: 'XX',
0|bh       |         password: 'X',
0|bh       |         native_parser: true },
0|bh       |      _applicationClosed: false,
0|bh       |      slaveOk: false,
0|bh       |      native_parser: true,

//db.js
var db = mongoskin.db(
	'mongodb://' + mongodb.username + ':' + mongodb.password + '@' + mongodb.host + ':' + mongodb.port + '/' + mongodb.databaseName,
	{
		safe: true,poolSize:8,strict: true
	}
);

As i already set the options in db.js,the output log shows that the serverConfig poolSize is still 1,how can i set a larger poolSize?