MV container and draw bitmap

Hi everyone…
here is my issue :

I have a MV called container. It has no specified size. I add to this container a child, a loaded SWF. And then, later, I draw this container into a BitMapData.

var jpgSource:BitmapData = new BitmapData (container.width, container.height);

On most of my loaded SWF, bitmap is the correct size of the original loaded swf. But sometimes, the bitmap is smaller and I miss part of my draw…

Does a my container take the size of his child ?