Camera.Render() Memory consumption

Dear Unity-Crew,

I have tracked a weird behavior in my application.
When I click a button, I render a texture using this function (this is a stripped down version, that does not save the rendered screenshot, but I have only posted the important part here):

Code (CSharp):

1.         IEnumerator takeScreenShot(ScreenShotEncodingType screenshotType, string postfix = "", bool automatic = false)
2.         {          
3.             Rect cropArea = CanvasUIManager.Instance.GetCropRect();
4.             Vector2 renderSize = CanvasUIManager.Instance.GetRenderSize();
5.             float scaleFactor = renderSize.x / cropArea.width;
6.  
7.             RenderTexture rt = RenderTexture.GetTemporary((int)Mathf.Ceil(Screen.width * scaleFactor), (int)Mathf.Ceil(Screen.height * scaleFactor), 24);
8.  
9.             Camera.main.targetTexture = rt;
10.             Camera.main.Render();
11.             Camera.main.targetTexture = null;
12.  
13.             RenderTexture.ReleaseTemporary(rt);
14.             RenderTexture.Destroy(rt);
15.             System.GC.Collect();
16.  
17.             this.CorrScreenshot = null;
18.  
19.             yield return new WaitForEndOfFrame();
20.  
21.         }

The problem is, that somehow the RenderTexture does not seem to be removed from memory completely.
The Application starts, then I click on the button, then a huge chunk of memory is allocated and released after the rendering but around 1/3 of the newly allocated memory is not freed up again.
When I comment the line that assigns the render texture to the camera targettexture nothing is created or allocated as I expected so it has to be the part when the render texture gets assigned to the camera and the camera finally renders into this texture.

I can't understand this behavior especially cause every example I could find uses Render Texture in this way. I even added the GC Collect without any success.