Trying to do the rubber effect on a button

1

I have this code on a instance of a button named myButton, but when I rollover it just expands without the effect. Also, why doesn’t using “this” instead of the instance name work the same?

on (rollOver) {
mybutton._width=100;
myButton._height=100;
mybutton._width=400;
myButton._height=400;
mybutton._width=200;
myButton._height=200;
}

Thanks.