Flash root Movie Resize Problem

Greetings ladies and gentlemen,

I have been going round and round in circles, trying to solve this annoying little problem, and have just ended up biting myself in the small of the back. I need help from someone who really knows their flash.

My problem is this - I have a parent flash movie of size 900x800. I have a button inside this parent that then loads an external swf (size 300x200) into an empty mc in the parent. My problem is that when the external swf loads, it resizes the parent movie (or repositions it depending on whether I am using IE or Firefox).

I can't seem to work out why or how it's happening, and it happens whether I use the standard Flash embedding or the swfobject method for my HTML page that the movie is embedded in.

The actionscript code in the parent is very basic:

but1.onRelease = function() {
_root.createEmptyMovieClip("mp3_player", getNextHighestDepth());
mp3_player._x = 240;
mp3_player._y = 390;
mp3_player.loadMovie("mp3player.swf");
}

I can't imagine why the root movie jumps down in size when the button is pressed and the external swf loaded. Please, could anyone shed some light?

My html file has the following embedding: