memory always goes up on loading tuxture

i use www to dynamiclly load texture in unity flash,
host swf will sendMessage to unity swf to load texture.
i try to release memory by destroying the asset,
but memory always go up after several load calls.
if switch to an empty scene memory doesn't change
if switch back to main scene memory doesn't change.
Resources.UnloadUnusedAssets(); and System.GC.Collect(); don't help.
who to manage memory?

Code (csharp):

1.  
2.     IEnumerator LoadUniformPNG(string name)
3.     {
4.         string path;
5.         if (Application.platform == RuntimePlatform.FlashPlayer)
6.             path = "StreamingAssets/" + name + ".png";
7.         else
8.             path = "file:///" + Application.streamingAssetsPath + "/" + name + ".png";
9.         Debug.Log(path);
10.         WWW www = new WWW(path);
11.         yield return www;
12.         Debug.Log("www png finish");
13.         DestroyImmediate(player.renderer.material.mainTexture, true);
14.         player.renderer.material.mainTexture = www.texture;
15.         www = null;
16.     }
17.  

btw, memory is ok in editor