When change view pager width and height through ViewGroup.LayoutParams,Animation doesn't work!

when I add this code to change the view pager width and height,the animation dosent work !

    DisplayMetrics dm = new DisplayMetrics();
    getWindowManager().getDefaultDisplay().getMetrics(dm);

    int w = dm.widthPixels;
    int h = dm.heightPixels;
    ViewGroup.LayoutParams layoutParams = mJazzy.getLayoutParams();
    layoutParams.width = w - h / 4;
    layoutParams.height = layoutParams.width * 3 / 2;