I'm looking around at top quilts, and they seem to range in width from about 48" to 55", with most I've seen at 50". Much of what I've read about the benefit of a top quilt vs. a sleeping bag partially unzipped and used as a quilt is that insulation that normally goes beneath you is instead used on top -- or alternatively that less insulation and fabric offers a weight savings.

I just measured by semi-mummy 20-year old synthetic sleeping bag, which I've been using for the past year as a top quilt. At the top, it's 54" wide -- which seems to suggest a pretty small savings given the potential disadvantages of not being able to use the top quilt as a sleeping bag for tent camping (which I need to do when camping with the family, at least for now). It seems maybe the zipper and hardware may contribute the most to the weight savings rather than less fabric or insulation?

So, in terms of weight savings and size vs. a traditional full mummy-stile sleeping bag (which should be a bit narrower still than my semi-mummy), the differences seem small, at least when comparing apples to apples in sleeping bag vs. top quilt. What am I missing? How much smaller will a comparable top quilt pack, and how much less should it weigh, than a similar sleeping bag?

Thanks for your thoughts as I decide between upgrading my 20* synthetic sleeping bag to either a 20* down sleeping bag or a 20* down top quilt.