There is cotton broadcloth and there is cotton/poly blend broadcloth. Kona Cotton Solids and Moda Bella Solids are both 100% cotton broadcloths with a thread count of 60. Quilt shop quality cotton prints are in the 68 to 75 thread count range. That's why the solids feel looser to you. Prewashing tightens the weave if that's what you want.
Broadcloth does not refer to the fiber content, it refers to the manufacturing method.