Flash Player 6.
my_btn._xscale
Property; the horizontal scale (percentage) of the button as applied from the registration point of the button.The default registration point is (0,0).
Scaling the local coordinate system affects the _x and _y property settings, which are defined in pixels. For example, if the parent movie clip is scaled to 50%, setting the _x property moves an object in the button by half the number of pixels as it would if the movie were at 100%.
Button._x, Button._y, Button._yscale