Hi, Tthe MovieMaterial does not use the actual MovieClip but rather an image of it. The dimensions of the bitmap is based on the actual size of the MC, so a small MC will have a lower bitmap resolution than a larger one. The dimensions are measured on the MovieClip when you create the material and they don't update after that.
You can however set the Width and Height manually using the material.lockW and material.lockH properties in either the init object when creating the material or as a property on the material. You can update it at any time but that will reduce the playback-speed of your project somewhat since that requires discarding and recreating bitmaps to the new dimensions. In other words - you can loose the extra layer of alpha by setting the correct lockW and lockH initially but otherwise this is how to do it. J
