Smooth Terrain Increase

Hi guys,

I have written a script for modifying the terrain level at runtime. Unfortunately, however, the terrain increases as a square instead of the desirable smooth circle, like in the built in terrain modification script.

 using UnityEngine;
 using System.Collections;
 
 public class TerrainEdit : MonoBehaviour {
 
     public Terrain terr;
     public int size = 20;
     // Use this for initialization
     void Start() {
 
     }
     
     // Update is called once per frame
     void Update() {
         if (Input.GetButton("Fire1") | Input.GetButton("Fire2")) {
             editTerrain ();
         }
     }
 
     void editTerrain() {
         int posX = (int)(((transform.position.x/terr.terrainData.size.x)*terr.terrainData.heightmapWidth)-(size/2f));
         int posZ = (int)(((transform.position.z/terr.terrainData.size.z)*terr.terrainData.heightmapWidth)-(size/2f));
         int xResolution = size;
         int zResolution = size;
         float[,] heights = terr.terrainData.GetHeights (posX, posZ, xResolution, zResolution);
 
         float increase;
 
         if (Input.GetButton("Fire2")) {
             increase = -0.001f;
         } else {
             increase = 0.001f;
         }
 
         int centX = posX - (size / 2);
         int centZ = posZ - (size / 2);
 
         for (int z = 0; z < zResolution; z++) {
             for (int x=0; x < xResolution; x++) {
                 float idealHeight = heights[x, z] + increase;
                 float dist = 1;
                 float mult = 1;
                 if (dist > 1) {
                     mult = 1/dist;
                 } else if (dist > size/4) {
                     mult = 0;
                 }
                 heights[x, z] = idealHeight * mult;
             }
         }
 
         terr.terrainData.SetHeights(posX, posZ, heights);
     }
 }
 

Any help would be greatly appreaciated