I think this is because pork loin roasts are cylindrical. They’re essentially the same thickness no matter how much they weigh. The differences in weight come from how long the roast is. An 8-pound pork loin roast will be the same circumference as a 4-pound roast but it will be twice as long. Cooking times for roasts are calculated to figure out how long it takes the heat to penetrate from the outside to the inside. Thus, it’s not going to take that much longer for the heat to get to the middle of the 8-pounder than the 4-pounder since they have the same distance from the side edges to the middle.