Gosh. Good question.
I didn't stop to think about it, but in this case I don't believe half racks will take any longer. On yesterday's 3 racks of ribs, one rack was cut in half cause it was placed into a Foodsaver bag and frozen. The half racks finished in the same time as the full racks.
So what about weight. I guess with ribs, weight refers to the amount of meat on the bones. Cutting them in half did not affect cook time for me.