Camera FOV bug or intended behaviour?

I want my object to cover all the Viewport. There is a simple formula using arctan to calculate the box scale depending of the camera FOV and the distance. The more FOV is the more space i get between box and the view margins. Why? Is it Mathf.Atan fault or the Unity camera behaviour?

Code (csharp):

1.  
2. public class TestFOV : MonoBehaviour
3. {
4.  
5.     private Camera cam;
6.     public Transform cube;
7.  
8.     private float h;
9.  
10.     void Start()
11.     {
12.         cam = GetComponent<Camera>();
13.     }
14.     void Update()
15.     {
16.         h = Mathf.Abs(cam.transform.position.z - cube.position.z);
17.  
18.         float hFOVinRad = cam.fieldOfView/2f*Mathf.Deg2Rad;
19.  
20.         float width = h*Mathf.Atan(hFOVinRad)*2f;
21.  
22.         cube.transform.localScale = new Vector3(1,width,.1f);
23.     }
24. }
25.  

upd. float width = h*Mathf.Atan(hFOVinRad)*2f; should be Tan sorry for bothering

 

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