Rear brake uncertainty

The backing plates are also matched to the shoes, and are specific to a shoe size.
The backing plate locates the wheel cylinders and hence the pushrod hardware.The pushrod centerline distance to the brakeshoe guideways determine 1/2 of the shoe backing plate width. Since I have never seen brake shoes with offset center-ribs, this then implys that the brake shoe would be 2 x the 1/2 width.
Now as the brake shoe width increases the backing plate has to move inboard, so that the matching drum does not rub on it.
So getting back to your question; If the drum is not rubbing on the backing plate, and has room for more shoe, then it likely has the wrong shoes installed. The proof is in the examination of the pushrod angle.If the shoes are too skinny, the pushrods will be running at an angle relative to the backing plate inboard wall or in fact to the car centerline.And that would be bad, as it would tend to rotate the shoes and wear off one edge faster than the other.