Combine meshes result in a larger file size on export (GLTF)

Hi @Bunny83 , thanks for the answer. The script to combine meshes is almost the same as the one provided by Bunzaga on this thread:

  using UnityEngine;
  using System.Collections;
  
  public class MeshCombiner : MonoBehaviour
  {
      public void Awake()
      {
          ArrayList materials = new ArrayList();
          ArrayList combineInstanceArrays = new ArrayList();
          MeshFilter[] meshFilters = gameObject.GetComponentsInChildren<MeshFilter>();
  
          foreach (MeshFilter meshFilter in meshFilters)
          {
              MeshRenderer meshRenderer = meshFilter.GetComponent<MeshRenderer>();
  
              if (!meshRenderer ||
                  !meshFilter.sharedMesh ||
                  meshRenderer.sharedMaterials.Length != meshFilter.sharedMesh.subMeshCount)
              {
                  continue;
              }
  
              for (int s = 0; s < meshFilter.sharedMesh.subMeshCount; s++)
              {
                  int materialArrayIndex = Contains(materials, meshRenderer.sharedMaterials[s].name);
                  if (materialArrayIndex == -1)
                  {
                      materials.Add(meshRenderer.sharedMaterials[s]);
                      materialArrayIndex = materials.Count - 1;
                  }
                  combineInstanceArrays.Add(new ArrayList());
  
                  CombineInstance combineInstance = new CombineInstance();
                  combineInstance.transform = meshRenderer.transform.localToWorldMatrix;
                  combineInstance.subMeshIndex = s;
                  combineInstance.mesh = meshFilter.sharedMesh;
                  (combineInstanceArrays[materialArrayIndex] as ArrayList).Add(combineInstance);
              }
          }
  
          // Get / Create mesh filter & renderer
          MeshFilter meshFilterCombine = gameObject.GetComponent<MeshFilter>();
          if (meshFilterCombine == null)
          {
              meshFilterCombine = gameObject.AddComponent<MeshFilter>();
          }
          MeshRenderer meshRendererCombine = gameObject.GetComponent<MeshRenderer>();
          if (meshRendererCombine == null)
          {
              meshRendererCombine = gameObject.AddComponent<MeshRenderer>();
          }
  
          // Combine by material index into per-material meshes
          // also, Create CombineInstance array for next step
          Mesh[] meshes = new Mesh[materials.Count];
          CombineInstance[] combineInstances = new CombineInstance[materials.Count];
  
          for (int m = 0; m < materials.Count; m++)
          {
              CombineInstance[] combineInstanceArray = (combineInstanceArrays[m] as ArrayList).ToArray(typeof(CombineInstance)) as CombineInstance[];
              meshes[m] = new Mesh();
              meshes[m].CombineMeshes(combineInstanceArray, true, true);
  
              combineInstances[m] = new CombineInstance();
              combineInstances[m].mesh = meshes[m];
              combineInstances[m].subMeshIndex = 0;
          }
  
          // Combine into one
          meshFilterCombine.sharedMesh = new Mesh();
          meshFilterCombine.sharedMesh.CombineMeshes(combineInstances, false, false);
  
          // Destroy other meshes
          foreach (Mesh oldMesh in meshes)
          {
              oldMesh.Clear();
              DestroyImmediate(oldMesh);
          }
  
          // Assign materials
          Material[] materialsArray = materials.ToArray(typeof(Material)) as Material[];
          meshRendererCombine.materials = materialsArray;
  
          foreach (MeshFilter meshFilter in meshFilters)
          {
              DestroyImmediate(meshFilter.gameObject);
          }
      }
  
      private int Contains(ArrayList searchList, string searchName)
      {
          for (int i = 0; i < searchList.Count; i++)
          {
              if (((Material)searchList[i]).name == searchName)
              {
                  return i;
              }
          }
          return -1;
      }
  }

I've also tried this payed solution from the Asset Store for combine meshes, with the same result on the export.