Memory Leak by www?

My test code here:

IEnumerator StartDownLoad(string url)
{
    WWW www = new WWW(url);
    yield return www;
    www.Dispose();
    www = null;
    Resources.UnloadUnusedAssets();
}

Run in webplayer(IE8/Firefox) and I find that :

after the function was called, the memory cost increased 30M+ in Windows Task Manager.

(the url points to a big assetbundle)

Why is the memory increased after dispose?

Is there any thing wrong in my code?.

(In Unity IDE, the memory show in Profile dose not changed after the function called)

I think you should try to use www.assetBundle.Unload(true);