Canvas calcSize question

[John Wiseman]

Why does canvas compute its size in this way, quadrupling height and doubling width and subtracting... 12?

Canvas.prototype.calcSize = function() {
  this.canvasSize = {width: this.width*2-12, height: this.height*4};
};

[John Wiseman]

Looks like @mvaladas subtracted 12 in commit cc4f597