First, the "they're sold out most of the time" soundbite is thrown around here all the time but never documented. I continually receive promotions from both travel consolidators and DCL offering discounted Disney sailings. That would not be happening if the ships were perpetually sold out. Am I saying DCL is hurting? No, but having been on the Magic in mid October and the Wonder in September I can tell you that that the ships are most certainly not always full.
Second, it is much, much easier to have high %s of full cabins when your fleet is small. The bigger you get, the harder that becomes and Disney is no different. If anything, they have a higher challenge to deal when adding capacity because the universe they target (people willing to pay a significant premium for a cruise) is finite, and the growing empty nest element of that market is less interested in the Disney "kids" positioning (which is why you find those people in spades on Celebrity and Cunard).
Again, when it's all said and done this boils down to ROI. The Iger mandate is to maximize capital returns. Unless cruises are the best way to do that, the incentive to put billions more into that business just doesn't exist.