Ever stare at your perfectly barbecued steak or ribs and wonder exactly what happens to the meat as it is being cooked?
When you BBQ meat you're breaking down the collagen in it by a slow chemical process. That's why it gets tenderized at the end. The collagen is what normally holds the tissues together, and the smoke slowly denatures the enzymatic bonds and causes the meat to pull apart easily. Essentially bbqing is like unraveling a sweater made out of protein. You're grabbing a thread and slowly pulling it apart.
When you grill something you're not allowing for even dispersion of the heat, but when you BBQ properly the thermal process is carried out evenly by convection almost the opposite of grilling
The heat goes from the inside to the outside, and on the outside, the myoglobin that was part of holding the meat together joins with nitrites in the muscle tissue and forms a bright pink chemical called nitrosohemochrome.
It's also why you have to cook bigger pieces of meat for longer, and why ribs are often bbqed.
The bones inside the ribs, when properly heated over time, create a secondary heat source. The ceramic (bone) is good at heating to a constant temperature and maintaining that heat over time.
So with ribs, you can have 12 lbs or 3 lbs, and their won't be much variation in cooking time.
Unlike with a roast, where the amount of heat is proportional to the weight of the meat. They don't have, essentially, heat sources throughout the meat.