i don't go by weight, but i go by girth of my snakes. does a medium rat equal (more or less) the girth of your BP? I think a happy belly shows a nice lump in your BP.
on the other hand, from an economical/efficiency standpoint if I had 20 small rats i would probably try to feed it all off before moving up in size. And i would pay attention if your BP is patrolling/hunting in between feeds with the small rats.