UnityWebRequest won't release memory after calling dispose

I use UnityWebRequest to download a picture( https://graph.facebook.com/4875021559178029/picture?height=256)

I try running the function 200 times to test UnityWebRequest, see below

public static void GetSpriteFromUrl(string _url, Action<Sprite> _cb)
{
UnityWebRequest request = UnityWebRequest.Get(_url);
UnityWebRequestAsyncOperation asyncOperation = request.SendWebRequest();
asyncOperation.completed += (asyncOperation) =>
{
/*
byte[] result = request.downloadHandler.data;
Sprite s = SpriteConvert.GetSprite(result);
_cb?.Invoke(s);
*/
request.downloadHandler.Dispose();
request.Dispose();
};
}

I comment out the callback part, so while downloading is completed it will call Dispose, and should be no other reference. But it turns out lose huge memory.

before



after


I have no idea which part I doing wrong.
Many thanks

 

it works! really appreciate!