ActionScript 3.0 :: Increasing Line Thickness Reduces Size Of Box

I made a small box that is filled with navy blue and has a red line around it, but when I increase the thickness to say 50, it reduces the size of my navy blue box.

PHP Code:
public class Box extends MovieClip {
public var color:uint = 0x000099; // class property of navy blue
public function Box() :void {
initialize();
construct();
this.graphics.lineStyle(50, 0xFF0000);
this.graphics.beginFill(color);
this.graphics.drawRect(0, 0, 100, 100);
this.graphics.endFill();
}