You’re running into a basic law of sleeping bag physics: The Law of Conservation of Volume.
Briefly stated, this law posits that a bag that allows more room to move聴i.e., a rectangular or semi-rectangular sleeping bag聴must of necessity be bulkier and heavier than a mummy bag of an equal temperature rating. I’ve devised a very elegant mathematical formula that proves this law, but I couldn’t possibly fit it into this column. Suffice it to say that my mind is pretty damned beautiful, too, but you don’t seen Jennifer Connelly hanging around me, for some unknown reason.
So part of me says you’re out of luck. Still, I expect that you don’t need a bag that has a ton of insulation. Probably one with a rating of about 40 degrees would do it, right? If so, then a bag like The North Face’s Bright Sky 3D might work well. It’s a semi-rectangular bag, meaning it’s bigger than a mummy but not as a big as a full rectangular bag. It doesn’t have a hood, and it’s full wrap-around zipper lets you use it as a light blanked or as a sleeping bag. Rating is 40 degrees, price is $109, and weigh is two pounds, one ounce. You might also look at the Cross Mountain from Big Agnes ($129), another 40-degree semi-rectangular bag. Big Agnes bags save weight by removing insulation from the bottom, relying instead on insulation from a sleeping pad that slips into a sleeve. The downside to this system is that restless sleepers may find themselves pinned to one position by the bag. But it’s a good quality bag and weighs the same as the Bright Sky at two pounds, one ounce. An excellent roomy mummy is the Andromeda Strain bag from Integral Designs. It comes in “broad” sizes that are cut wider from the hip to the foot. It’s $210, weighs two pounds, and is rated to 40 degrees, although I believe it’s warmer than either the Big Agnes or Bright Sky in real-world applications. I’d also recommend a bag liner, which will add some warmth but more importantly keep the bag clean, extending its life because you’ll need to wash it less often. Design Salt’s Cocoon Silk Liner ($60) would be a great choice, and could even be used as a superlight bag when it’s very warm.