The new ZenBivy Ultralight Quilt has some improved specs that are approaching specs of the leading quilts. Here's a comparison to the EE Enigma and Katabatic Palisade, Alsek, and Sawatch with similar temp ratings (quilt only; definitely note that Katabatic lists comfort ratings, ZenBivy and EE list limit ratings). All of these are 54" width at the shoulder and 40" at the foot box:
The zen bivy is an excellent product. I currently have and use a Big Agnes Sidewinder sleeping bag that I like a lot. It's an excellent design that suffers from poor loft compared to better brands and a few other quirks. I sleep well in it and recommend it to side sleepers. Not saying it's a better option than the zen bivy, but it's another option to consider. Choices are a good thing to have.