How closely a down bag fits your body does have an extreme effect on its warmth. A bag that may be warm for a big guy, may freeze a smaller guy - why? Because empty airspace means more air to warm up between you and the bag. This may seem counter intuitive, after all isn't loft the primary thing? A bag that will keep you warm at its rating will most likely feel constricting. Since I HATE tight bags, but do go out in some pretty nasty weather I needed a solution.

My winter bag is a WM super Kodiak. Its rated at -5 but I was cold in it even at +5. However I weigh 175 and it was designed for 280 pound 6 foot 6 line backer. The extra space was simply too much for me warm up.

The solution. I have two thin 1/8 inch diameter elastic cords buried inside the bag that attach near the zipper and are sewed done after adjusting the tension so they just barely pull the bag against my body. There is a band near my waist and one near my knees. I can stretch out inside this huge bag if I choose, yet the elastic pulls it against me and now I am cozy at -5. There are now three air pockets in the bag and there are no drafts because the elastic prevents it. I think I may be the only person on the planet that does this and it is such a simple fix that I don't understand why others don't do it.
Jim S