Problem changing the size of an MC with AS

1

greetings everyone…
i am trying to change the size of an MC with this script:

onClipEvent(load){
  this.onRollOver = function(){
          this._height = this._height*2;
        this._width = this._width*2;
          }
} 
onClipEvent(load){
  this.onRollOut = function(){
          this._height = this._height/2;
        this._width = this._width/2;
          }
}

this code is on a MC… the problem is… that it had some motion animation with the frames… but now that it has this script, it doesnt animate…
i also kinda have been noticing that i should put the AS on the frames and not in the objects… why should i take that advice? is there any big diference?