@eric, no it won't kill him if it is, in fact, a warmer bag... which is what I'm trying to determine. He's a skinny 13yo and doesn't need the extra girth, so no point in paying the 4oz hit just for the size.
@Andrew, thanks for pointing out the WM example. Based on straight math, I figure that the extra 2" needs for about an ounce of down (2" x 4" loft x 77" length = 616 cubic inches.) So, theoretically, the other 2oz of down just adds warmth. Just not sure that's reality, however.