After you baste the quilt down, measure across the width of the quilt at that basting line. Before basting down the sides each time, measure across the middle and at the point nearest you to make sure that it is still that size.
My guess is that you are not tensioning the sides so that, when you baste the sides down, the quilt is not stretched as much side-to-side as when you basted the top down. It probably gets a little worse each time you roll. The backing is on rollers so this is not happening to the backing.
The problem is not that you are stretching the top too much; it is that you are not stretching the top enough. Tensioning the sides while you baste would help. Measuring before you baste ensures that you are not "scooching" the top into a smaller width with each roll.