if they won't supply you with a piece, ask them to tell you the weight per yard. to be honest they may not know it, but the maker does and they all have an 800 number. also, they may use more stiffener or other sizing in the 45" fabrics, and that would make it feel a lot heavier and weigh more, too. if top and backing felt like that, your needle would have to punch it's way through topping, batting and backing. that's one reason that muslin is so popular as a backing. even good muslin is less tight and heavy than good quilting fabric.
if the quilting fabric and the wide backing were both washed, they might feel the same or at least much closer in weight.
ask for thread count/weight per yard. let us know.