The thickness of the meat that affects cooking time more so than the length or width. If you take a 3" steak and slice it into two 1 1/2" steaks, they will cook far more quickly.
Cutting a 3" steak weighing 3 pounds into two steaks still 3" thick, but weighing 1 1/2 pounds each will take nearly as long to cook as the original steak. There will be a slight difference, but not nearly as much as you might think.
With the baby back ribs, cutting a full rack into two half-racks won't affect cooking time much at all, but if you were to cut the rack into individual riblets containing one bone each, the cook time would be decreased significantly. As the riblets will be exposed to smoke and heat on more surface area.