If all you do when you make a quilt is basically leave off the zipper then sure, it will only weight a few ozs less than the bag.
If you want to make a real comparison you need to weigh a bag that works for you at the lowest temp you want and then weigh a quilt that works for you at the same temp. This is a real involved and expensive process, but gives an accurate comparison. If your quilt ends up being that same size as the bag, well then it's not going to weight much less. However, if it ends up being smaller then it will.
Personally, I think the real advantage of a quilt is that it a much more comfortable sleep with no weight penalty. It takes all that useless insulation and puts it somewhere I can use it, namely to spread out!
As always, everyone is different. Quilts work better for some, bags for others.