Several instances destroyed when one is shot by player

I don't suspect anyone will be able to help me here, but here it goes.

I have an asteroid script that chooses a size(small, medium or large) at random when the game starts. The size variables are set in an enumerator. This created a problem when destroying a large asteroid, because the two new asteroids instantiated picked random sizes, even large, when I needed two medium asteroids. To solve this, I created another enumerator, prevAsteroid, and a preLoadedAsteroid boolean.

Everything seemed to work perfectly, until I noticed a bug. The large asteroid splits into two medium asteroids. Upon shooting the a medium asteroid, it breaks into two small asteroids, as I had intended. However, if the medium asteroid had come from a large asteroid, then its "sister asteroid" also affected. It turns right into another small asteroid, and there are three total.

I know the location of the bug, but I don't know how to fix it. Here is the location of the bug.

 } else if (!sizeChosen && !preloadedAsteroid) 
         {
             switch (prevAsteroid)
             {
                 case PrevAsteroid.Small:
                     break;
                 case PrevAsteroid.Medium:
                     asteroidSize = AsteroidSize.Small; //right here. This effects both medium asteroids that were instantiated when the large asteroid was destroyed. 
                     break;
                 case PrevAsteroid.Large:
                     asteroidSize = AsteroidSize.Medium;
                     break;
             }
         }
 
     }
   
     void OnTriggerEnter(Collider other)
     {
         if (other.gameObject.name == "BlasterBolt(Clone)")
         {
             switch (asteroidSize)
             {
                 case AsteroidSize.Small:
                     prevAsteroid = PrevAsteroid.Small;
                     Instantiate(dustObject, transform.position, Quaternion.identity);
                     break;
                 case AsteroidSize.Medium:
                     prevAsteroid = PrevAsteroid.Medium;
                     Instantiate(asteroidObject, transform.position, Quaternion.identity);
                     Instantiate(asteroidObject, transform.position, Quaternion.identity);
                     Instantiate(dustObject, transform.position, Quaternion.identity);
                     break;
                 case AsteroidSize.Large:
                     prevAsteroid = PrevAsteroid.Large;
                     Instantiate(asteroidObject, transform.position, Quaternion.identity);
                     Instantiate(asteroidObject, transform.position, Quaternion.identity);
                     Instantiate(dustObject, transform.position, Quaternion.identity);
                     break;
             }
             Destroy(gameObject);
             Destroy(other.gameObject);
         }

While I wrote this script without any help, it was pushing the limits of my brain, and I don't even fully understand everything I did, so I'm not sure I have given enough information. Any ideas?