For a long time, I thought Warm and Natural was 100% cotton, but it's actually 70% cotton/30% polyester. It's right on the label that's rolled up in the batting.
One reason that quilters like 100% cotton batting, is that it will shrink and make the quilt "crinkle,"like an antique quilt. A batting with a little polyester won't shrink as much.
I like my batting a little thicker, so I've been using Winline 80/20 Comfort Blend, 6 oz, which is double what the regular Comfort Blend is. With a thicker batting, I don't want to quilt too densely.
I agree with LAF2019 that the stiffness comes from the density of the quilting.