spawn bigger objects at times

I'm making a asteroid game where asteroid come in from each side of my game. There's no problem with my spawning script i just want to know how can i make my spawning script spawn bigger objects randomly. You know it spawns my object small then it spawn big ones sometimes but still spawning the small ones. (Sorry if I'm not making sense right now) here's my script:

 using UnityEngine;
 using System.Collections;

  public class Sidespawn : MonoBehaviour {
 
 public GameObject ObjectToSpawn;   
 public float RateOfSpawn = 1;
 public float SpawnDelay = 20f;
 private float nextSpawn = 0;
 
 // Update is called once per frame
 void Update () {           
     
     if (Time.time > nextSpawn) {
         nextSpawn = Time.time + RateOfSpawn;
     }
 }
 void Start(){
     Invoke("Spawn", SpawnDelay);
 }
 
 void Spawn () {           
     // Random position within this transform
     Vector3 rndPosWithin;
     rndPosWithin = new Vector3(Random.Range(-1f, 1f), Random.Range(-1f, 1f), Random.Range(-1f, 1f));
     rndPosWithin = transform.TransformPoint(rndPosWithin * .5f);
     Instantiate(ObjectToSpawn, rndPosWithin, transform.rotation);  
     
     Invoke("Spawn",SpawnDelay);
 }

}