How to get expanding object collide when it touches others?

I have an object that is expanding by adding to its localscale each frame. I want it to stop expanding when it hits another collider, but OnCollisionEnter never fires (neither does OnCollisionExit, or OnCollisionStay). How can I tell it to trigger something once it expands to touch another collider?

Here's my script:

 using UnityEngine;
 using System.Collections;
 public class script_blob_player : MonoBehaviour {
     
     private bool touchingSomething = false;

     void Update () {
         if (!touchingSomething)    {    Expand ();    }
     }

     void Expand () {
         float expansionFactor = 1.0f;
         transform.localScale += new Vector3(expansionFactor, 0, expansionFactor) * Time.deltaTime;
     }
     
     void OnCollisionEnter (Collision collision) {
         touchingSomething = true;
     }
 }
 

And yes, my growing object does have a rigid body, which appears to grow as my object grows. And if I turn on gravity so that it falls on another object, the OnCollisionEnter DOES fire. But I'm not using gravity here.

I also found an old similar question that was never answered: http://answers.unity3d.com/questions/142211/detect-collision-between-static-and-expanding-obje.html

Thanks!