This can be done very easily by adjusting the x and y of the speech bubble by half of the growth amount. For example, if you are scaling the object by 10 pixels, you would move the sprite to the right and up by 5 pixels. To show you what I mean, I am attaching a very simple capx file for you.
When you run the cap, all you have to do is click the red square. It will stretch diagonally up and to the right. You can click it again and it will shrink in the opposite direction.
The important thing is to remember you have to adjust the position by exactly half the number of pixels you stretch it by.