Yes, the neck is too loose on Will. Will’s garment reviews are my favorite BPL content. He analyzes garments differently than I do but, I never fail to gain valuable additional insights from his reviews. After his highly recommended review of the WM Hooded Flash, I just had to try one on… I hated it. In a large measure this was because it fit me very differently than it fit Will. The size that maximized my torso warmth (see below), had hood dimensions that were totally ineffective. Compounding that, there were no adjustments to alter any aspect of the fit. It was my most recent reminder that “size matters”.
For any jacket to achieve its maximum insulation potential the neck has to seal, as does the wrists, and waist. Otherwise you have "chimney effect" heat loss when inactive. As your body heats air around your torso, it rises and goes out your neck. This in turn actively draws cold air up from the openings in the waist and wrist area.
You want the body of the jacket to be loose so as to maximize the garment warmth. The air space between the jacket and your next inner layer acts as additional insulation up to a max of about 3/4”. If you are moving, then the “billows effect” will pump both excess moisture and warmth out of your jacket. This is the reason that a down jacket doesn’t loose its insulating value in environments which it would be less than optimal for a down sleeping bag. For UL backpacking you normally only wear your insulation layer when your relatively stationary. Consequently, the warmth doesn’t billow out when you most need it; for example cooking breakfast or dinner. As you become more active, then your MET rate goes up and you want the billows effect to extend your thermal neutral range.
I alter the neck and wrists of many of my jackets to achieve their full warmth potential. All that is required is a needle, some polyester or nylon thread, and about a half hour of time. A neck looks best if it is taken in at both shoulder seams. If you take it in by using a single seam in the back of the neck, you will get a "hump back" material blossoming below the seam. Pin it and then try it on to insure you have the proper size prior to sewing.
The insulation value of an air space falls off rapidly when the air space is any measure thinner than 3/16 inch. The thickness of the insulating air films on each surface is reduced. The R value increases up to about ¾ inch when air convection in this space begins to offset any increase in distance between the surfaces. Although not relevant to jacket fit, the curve remains fairly level in this range with the insulation value near 1 until the curve exceeds 4 inches. Beyond 4 inches, convection currents disturb the air films and cause the R value of the air space to decrease.