Why is new Vector3 stronger when player is in the air?

I am trying to make a knock back script. When I walk into enemy the while grounded the player gets slightly bumped, but when I jump and hit the enemy the player goes much farther.

Here is my code (I have a script on the enemy to cause knock back and one on the player for resetting):

 public float xVel = 15f;
 public float yVel =  7f;
 public bool KnockedBack = false;
 private GameObject otherPlayer;

 void OnTriggerEnter (Collider other)
 {
     if (other.gameObject.tag == "Player")
     {    
         other.gameObject.GetComponent<Rigidbody> ().velocity = Vector3.zero;
         other.gameObject.GetComponent<Rigidbody> ().angularVelocity = Vector3.zero;
         other.gameObject.GetComponent<Rigidbody> ().velocity = new Vector3 (-xVel, yVel, 0f);
         KnockedBack = true;
         other.gameObject.GetComponent<ResetKnockBack> ().grounded = false;

         print (other.gameObject.GetComponent<ResetKnockBack> ().grounded);

     }
 }

}

 public bool grounded = true;
 public GameObject opponent;

 void OnCollisionEnter (Collision other)
 {
     print (grounded);

     if (other.gameObject.tag == "Ground")
     {    
         StartCoroutine (resetTime ());

     }

     if (grounded && opponent.GetComponent<KockbackPlayer> ().KnockedBack == true)
     {
         other.gameObject.GetComponent<KockbackPlayer> ().KnockedBack = false;
         this.gameObject.GetComponent<Rigidbody> ().velocity = Vector3.zero;
     }
 
         
 }



 IEnumerator resetTime ()
 {
     yield return new WaitForSeconds (0);
     //grounded = true;

 }

}