AS3 Why does editing a Textfield inside of a MovieClip throw all the sizes off? - actionscript-3

Question

I'm pretty frustrated with AS3 right now. It seems like there must be serious issues with how things scale inside of movieclips. From the stage everything seems to work fine.
On one of my projects I try to set a textfield.width equal to its container(movieclip)
tf.width = mc.width; Of course it says everything is even but when I look at the border of the textfield it's nowhere near the size of the movie clip it's contained within.
While trying to make a much smaller sample to share with you guys for help, that part worked, but trying to resize text did something completely different. Can anyone help me make sense of all this? The code below seemingly randomly starts changing the size of everything.
Also, I was trying to follow this code from here but just kept getting an error when trying to change the format size in a while loop: Autosize text to fit the width of a button
var square:MovieClip = new MovieClip();
addChild(square);
square.graphics.lineStyle(3,0x00ff00);
square.graphics.beginFill(0x0000FF);
square.graphics.drawRect(0,0,100,100);
square.graphics.endFill();
square.x = 0;
square.y = 0;
square.width = 200;
square.height = 200;
var tffSize = 400;
var tffOr:TextFormat = new TextFormat();
tffOr.size = tffSize;
tffOr.align = TextFormatAlign.CENTER;
var tf:TextField = new TextField();
square.addChild(tf);
tf.defaultTextFormat = tffOr;
tf.text = "Hello";
tf.border = true;
tf.width = square.width;
tf.height = square.height;
trace(tf.textWidth);
trace(square.width);
while (tf.textWidth > square.width || tf.textHeight > square.height)
{
trace("too Big");
newTFF();
trace(tf.textWidth + " vs " + square.width);
square.width = 200;
trace(tf.textWidth + " vs " + square.width);
}
function newTFF()
{
tffSize = tffSize - 1;
var tff:TextFormat = new TextFormat();
tff.size = tffSize;
tff.align = TextFormatAlign.CENTER;
//tf.defaultTextFormat = tff;
tf.setTextFormat(tff);
tf.autoSize = "left";
}