Which, in turn, is another way of saying that the only straightforward comparison is between the seat-mile costs of same-sized aircraft (and even that doesn't take into account factors like range, comfort and passenger appeal).
As soon as you try to make comparisons between differently-sized aircraft, you usually end up with the larger aircraft having lower SMCs, but higher aircraft-mile costs.
Which is great provided you can fill your aircraft, but not so good if half your potential passengers want to travel at a different time of day.