Why do I get this weird mesh error when I have a mesh over 250 by 250?

I am trying to generate a mesh that is over 250 by 250, but for some reason it breaks and it does not work, it gets weird artifacts on the mesh. If try doing it the mesh for some reason stays rectangular though it should be a square because I only use square sizes so I am sure the error has to do with the rest of the mesh.

 using System.Collections;
     using System.Collections.Generic;
     using System.Security.Cryptography;
     using UnityEngine;
     using UnityEngine.AI;
     using UnityEngine.UIElements;
     
 public class MeshGenerator : MonoBehaviour
 {   //Resolucion del terreno
     public int xSize;
     public int zSize;
     float minTerrainHeight;
     float maxTerrainHeight;
     public int offsetX;
     public int offsetZ;
     //Modificador de aleatoriedad
     public float RandomModifier;
     //Color del terreno generado
     Color[] TerrainColor;
     //Mesh
     Mesh mesh;
     //Vertices y cordenadas de vertices
     Vector3[] vertices;
     int[] triangles;
     //Color del terreno
     public Gradient TerrainHeightColor;
 
     void Start()
     {
         mesh = new Mesh();
         GetComponent<MeshFilter>().mesh = mesh;
         CreateShape();
         UpdateMesh();
         Debug.Log("RandomModifier = " + RandomModifier);
 
     }
     void Update()
     {
     }
     void CreateShape()
     {
         vertices = new Vector3[(xSize + 1) * (zSize + 1)];
         //Generador de las cordenadas de cada vertice del triangulo
         for(int i = 0,z = 0; z <= zSize; z++)
         {
             for(int x = 0; x <= xSize; x++)
             {
 
                 int y = Mathf.RoundToInt(Mathf.PerlinNoise((x+ offsetX) * 0.009f + RandomModifier,(z+offsetZ)*0.009f + RandomModifier)*40f);
                 vertices[i] = new Vector3(x,y,z);
                 i++;
 
                 if (y > maxTerrainHeight)
                     maxTerrainHeight = y;
                 if (y < minTerrainHeight)
                     minTerrainHeight = y;
                 //Debug.Log("y = " + y);
 
             }
         }
 
 
         triangles = new int[xSize * zSize * 6];
         int vert = 0;
         int tris = 0;
 
         //Generador de vertices de los triangulos
         for(int z = 0;z < zSize; z++)
         {
             for(int x = 0;x < xSize; x++)
             {
                 triangles[tris + 0] = vert;
                 triangles[tris + 1] = vert + xSize +1;
                 triangles[tris + 2] = vert + 1;
                 triangles[tris + 3] = vert + 1;
                 triangles[tris + 4] = vert +xSize+ 1;
                 triangles[tris + 5] = vert + xSize + 2;
 
                 vert++;
                 tris += 6;
             }
             vert++;
         }
         TerrainColor = new Color[vertices.Length];
 
         for(int i= 0,z = 0;z <= zSize;z++)
         {
             for(int x = 0; x <= xSize; x++)
             {
                 float height = Mathf.InverseLerp(minTerrainHeight,maxTerrainHeight, vertices[i].y);
                 TerrainColor[i] = TerrainHeightColor.Evaluate(height);
                 i++;
             }
         }
         Debug.Log("MAX:" + maxTerrainHeight);
         Debug.Log("MIN:" + minTerrainHeight);
     }
     void UpdateMesh()
     {
         mesh.Clear();
         mesh.vertices = vertices;
         mesh.triangles = triangles;
         mesh.RecalculateNormals();
 
         for (int i = 0, z = 0; z <= zSize; z++)
         {
             for (int x = 0; x <= xSize; x++)
             {
                 float height = Mathf.InverseLerp(-1, 40, vertices[i].y);
                 TerrainColor[i] = TerrainHeightColor.Evaluate(height);
                 i++;
             }
         }
         mesh.colors = TerrainColor;
     }
     private void OnDrawGizmos()
     {   if (vertices == null) return;
         for(int i = 0; i < vertices.Length; i++)
         {
             Gizmos.DrawSphere(vertices[i], .1f);
         }
     }
 }