Why are my screenshots created through a RenderTexture too small?

Question

Current render result:

When I am rendering image in Unity and saving as file(.png), quality of those files are reduced. They become tiny and very poor.

How can I enhance that?

Dimension of the image file is 269*168.

Code to save image :

using UnityEngine;
using System.IO;
using System;

//getty code
public class Capture : MonoBehaviour
{
    private static int resWidth = 3840;
    private static int resHeight = 2160;
    private static GameObject camObj = null;


    public static string ScreenShotName()
    {
        return string.Format("{0}/screenshots/{1}.png",
                             Application.dataPath,
                             Util.name_);
    }

    void Start()
    {
        camObj = gameObject;
        Directory.CreateDirectory(Application.dataPath + "/screenshots");
        resHeight=GetComponent<Camera>().pixelHeight;
        resWidth=GetComponent<Camera>().pixelWidth;
        //Util.everyThingHot = true;
    }

    public static void TakeHiResShot()
    {
        try
        {
            RenderTexture rt = new RenderTexture(resWidth, resHeight, 24);
            camObj.GetComponent<Camera>().targetTexture = rt;

            Texture2D screenShot = new Texture2D(resWidth, resHeight, TextureFormat.RGB24, false);
            camObj.GetComponent<Camera>().Render();
            RenderTexture.active = rt;
            screenShot.ReadPixels(new Rect(0, 0, resWidth, resHeight), 0, 0);
            camObj.GetComponent<Camera>().targetTexture = null;
            RenderTexture.active = null;
            Destroy(rt);
            byte[] bytes = screenShot.EncodeToPNG();
            string filename = ScreenShotName();
            File.WriteAllBytes(filename, bytes);
            Debug.Log(string.Format("Took screenshot to: {0}", filename));
            bytes = null;
        }
        catch(Exception e)
        {
            Debug.Log("Error");
        }

    }

}

Sample:

What I see during saving shot:

It's taking a lot CPU,GPU resources, renders correctly but doesn't give me what I want as a file

Answer 1

The problem was I was changing the value of resHeight and resWidth as @DMGregory mentioned. So it was rendering in poor quality.

Now,

using UnityEngine;
using System.IO;
using System;

//getty code
public class Capture : MonoBehaviour
{
private static int resWidth = 3840;
private static int resHeight = 2160;
private static GameObject camObj = null;


public static string ScreenShotName()
{
    return string.Format("{0}/screenshots/{1}.png",
                         Application.dataPath,
                         Util.name_);
}

void Start()
{
    camObj = gameObject;
    Directory.CreateDirectory(Application.dataPath + "/screenshots");
    //resHeight=GetComponent<Camera>().pixelHeight; *Line changed*
    //resWidth=GetComponent<Camera>().pixelWidth; *Line changed*
    //Util.everyThingHot = true;
}

public static void TakeHiResShot()
{
    try
    {
        RenderTexture rt = new RenderTexture(resWidth, resHeight, 24);
        camObj.GetComponent<Camera>().targetTexture = rt;

        Texture2D screenShot = new Texture2D(resWidth, resHeight, TextureFormat.RGB24, false);
        camObj.GetComponent<Camera>().Render();
        RenderTexture.active = rt;
        screenShot.ReadPixels(new Rect(0, 0, resWidth, resHeight), 0, 0);
        camObj.GetComponent<Camera>().targetTexture = null;
        RenderTexture.active = null;
        Destroy(rt);
        byte[] bytes = screenShot.EncodeToPNG();
        string filename = ScreenShotName();
        File.WriteAllBytes(filename, bytes);
        Debug.Log(string.Format("Took screenshot to: {0}", filename));
        bytes = null;
    }
    catch(Exception e)
    {
        Debug.Log("Error");
    }

}

}

Now everything renders perfectly in 4K.