Problem Creating a 2D Mesh

I am trying to teach myself how to create custom meshes in Unity. I have started by trying to create a very simple 2d plane. So far I have managed to write some code that creates a 2x2 square, but when I try to increase the size of the plane (that is how many tiles comprise the plane) the compiler throws an index out of range exception. I haven't been programming for long and can't seem to understand why I can build a 2x2 mesh, but nothing larger or smaller. I'm new at this so there are probably a lot of mistakes. Any help would be greatly appreciated. Thanks.

 using UnityEngine;
 using System.Collections;
 
 [RequireComponent(typeof(MeshFilter))]
 [RequireComponent(typeof(MeshRenderer))]
 [RequireComponent(typeof(MeshCollider))]
 
 public class TileMap : MonoBehaviour 
 {
     
     //size of mesh in tiles
     public int sizeX = 2;
     public int sizeZ = 2;
     
     //size of each tile
     public float tileSize = 1.0f;
     
     // Use this for initialization
     void Start () 
     {
         BuildMesh();    
     }
     
     void BuildMesh()
     {
         //declare mesh variables
         int totalTiles = sizeX * sizeZ;
         int totalTris = totalTiles * 2;
         //vertices, uvs, and normals
         int numVertsX = sizeX + 1;
         int numVertsZ = sizeZ + 1;
         int totalVerts = numVertsX * numVertsZ;
         
         Vector3[] verts = new Vector3[totalVerts];
         Vector3[] norms = new Vector3[totalVerts];
         Vector2[] uvs = new Vector2[totalVerts];
         
         //triangles
         int[] tris = new int[totalTris * 3];
         
         //generate mesh data
         //vertices
         for(int z = 0; z < numVertsZ; z++)
         {
             for(int x = 0; x < numVertsX; x++)
             {
                 verts[z * numVertsX + x] = new Vector3(x * tileSize,0,z * tileSize);
                 norms[z * numVertsX + x] = Vector3.up;
             }
         }
         
         //uvs
         //no texture, just a simple red material
         uvs[0] = new Vector2(0,0);
 
 
         //triangles
         int i = 0;
         for(int z = 0; z < sizeZ; z++)
         {
             i = i + z;
 
             for(int x = 0; x < sizeX; x++)
             {
                 i = i + x;
 
                 tris[i] = (x * numVertsX) + z + 0;
                 tris[++i] = (x * numVertsX) + z + 3;
                 tris[++i] = (x * numVertsX) + z + 4;
 
                 tris[++i] = (x * numVertsX) + z + 4;
                 tris[++i] = (x * numVertsX) + z + 1;
                 tris[++i] = (x * numVertsX) + z + 0;
 
             }
 
         }
         
         //create a new mesh
         Mesh newMesh = new Mesh();
         
         //assign mesh to filter/collider/renderer
         MeshCollider meshCollider = GetComponent<MeshCollider>();
         MeshFilter meshFilter = GetComponent<MeshFilter>();
         MeshRenderer meshRenderer = GetComponent<MeshRenderer>();
         meshFilter.mesh = newMesh;
         
         //assign data to mesh
         newMesh.vertices = verts;
         newMesh.uv = uvs;
         newMesh.normals = norms;
         newMesh.triangles = tris;
         
     }
 }