SquishScale.cs

using UnityEngine;
using System.Collections;

public class SquishScale : MonoBehaviour
{
    public float amount = .1f;
    public float speed = 10f;
    public float timeOffset;
    private Vector3 originalScale;

    void Awake()
    {
        originalScale = transform.localScale;
    }

    void Update()
    {
        transform.localScale = new Vector3(
                originalScale.x + Mathf.Sin(Time.time * speed + timeOffset) * amount,
                originalScale.y + Mathf.Sin(Time.time * speed + timeOffset + Mathf.PI) * amount,
            1f);
    }
}