UI_ButtonScale.cs

using UnityEngine;
using System.Collections;
using UnityEngine.UI;
using UnityEngine.EventSystems;
[RequireComponent (typeof (Button))]
public class UI_ButtonScale : MonoBehaviour,IPointerDownHandler ,IPointerUpHandler {
    public AnimationCurve animCurve;
    [Tooltip("Animation speed multiplier")]
    public float speed = 1;
    private Vector3 initScale;
    public Transform target;

    Button button;
    public Button Button
    {
        get
        {
            if (button == null)
                button = GetComponent <Button> ();

            return button;
        }
    }
    // Use this for initialization
    void Awake()
    {
        if(target == null)
        target = transform;

        initScale = target.localScale;
    }
    void OnEnable()
    {
        target.localScale = initScale;
    }
    public void OnPointerDown(PointerEventData eventData)
    {
        if (Button != null && !Button.interactable)
            return;

        StopCoroutine("ScaleIN");
        StartCoroutine("ScaleIN");
    }
    public void OnPointerUp(PointerEventData eventData)
    {
        if (Button != null && !Button.interactable)
            return;

        StopCoroutine("ScaleOut");
        StartCoroutine("ScaleOut");
    }

    IEnumerator ScaleIN()
    {
        target.localScale = initScale;
        float t=0;
        float maxT = animCurve.keys[animCurve.length-1].time;

        while(t<maxT)
        {
            t+= speed * Time.unscaledDeltaTime;
            target.localScale = Vector3.one * animCurve.Evaluate(t);
            yield return null;
        }
    }
    IEnumerator ScaleOut()
    {
        target.localScale = initScale;
        float t=0;
        float maxT = animCurve.keys[animCurve.length-1].time;

        while(t<maxT)
        {
            t+= speed * Time.unscaledDeltaTime;
            target.localScale = Vector3.one * animCurve.Evaluate(maxT-t);
            yield return null;
        }
        transform.localScale = initScale;
    }
}