For Castaway, I'm pretty sure I read somewhere that the terms of the lease Disney has with the Bahamian government restricts their ability to develop more than 5-10% of the island or something like that. They are probably at max capacity which is why they went ahead and bought Lighthouse Point to begin with.Castaway Cay is technically undeveloped. Only 55 acres of the 1000 acres are currently being used. Compare that to Cococay that is 125 acres in total. You could also technically tender to the island with a bigger ship. Options are obviously there if Disney wanted them. And then of course we have no knowledge on what the plans are for Lighthouse Point. Its about 700 acres of land. I can see them going one of two ways. Making Lighthouse Point the smaller, more natural island and developing Castaway Cay for bigger ships. Or just making Lighthouse Point able to handle capacity of more guests. You could host multiple cruises or double the capacity at Lighthouse Point if you used only 1/7th of the island.
For Lighthouse, Disney committed to only develop 20% of the land, but that is still 140 acres so perhaps they are planning future capacity for more than 1 ship at a time.