You are absolutely correct that the top adds weight and the more piecing the heavier it is. Additionally not all quilting cottons weigh the same. I find most flannels feel heavier than regular cottons and every T-shirt quilt I have made has been very heavy so that lightweight fusible also adds weight and T-shirt knit is often heavier than woven cotton. There are no rules of thumb that I am aware of because there are too many variables. Some cottons weigh more than others. Smaller pieces mean more seam allowances and more thread. This will add weight. Is there applique? That can add to the weight as well, especially if the backing is not cut away from behind the applique. All the components of a quilt add to it's weight and there are different variables for all of them.
For a light weight warm batting with loft look to wool but wool batting for a queen size quilt will run you around $40 with no sale or coupons involved. A much less expensive option would be Hobbs Poly down or Quilters dream dreampuff batting.