Invisible button

I’m new to Flash, and have a problem I can’t solve.

I have a piece of graphic, which is 100 x 200 pixels, but I only want an area of 75 x 175 pixels of the graphic to function as a button. I have figured that one out how to solve it. Now here comes my real problem.

My mouseover movie clip is also 100 x 200 pixels, and everywhere it go, it makes the button area larger. How do I prevent that from happening, if it’s possible?

Do I have to make a button inside a button or…?