I need a warmer and wider bag than my 30 degree Marmot Hydrogen. The dimensions of the light, slim cut mummy bags at Western Mountaineering (WM) are smaller than my current bag, so I'm considering a 20 degree Alpinlite or a 25 degree Sycamore for the longer zippers and wider cut.

What makes the WM bags so much warmer than the competition? Is it the slim cut? The down is about the same quality as others, so does WM use more of it? Is the fabric more windproof? I assume the design is not a secret. Is it the US pride in craftsmanship?

The Sycamore with 2 oz. of free overfill is still $50 less than the Alpinlite. The bags are different widths, but that does equalize the fill weight. Is anyone using a wide cut WM bag and regrets needing to heat up the extra space on the inside?
Thank you for sharing your experiences and knowledge.
Joe