onClip(data)mc._width=12 not working properly

Hi guys.
Whats the deal with _width.
I am loading video(swf) into it’s own mc and using “onclipEvent(data)” to set width and height of the movie once it has loaded. The height works fine but the width does not. It only seems to effect swf movies. images load and dimenssions can be changed without any problems.
Anyone
THanks