Well, when you get to be a certain size, the materials used in the construction are going to be under so much load under just their own weight that they won’t be able to handle the stresses of flying, and since strength:weight is just about the biggest challenge when it comes to flying, you can’t just keep piling on composites to keep it strong as it gets bigger. There’s a reason all of the biggest and heaviest animals that have ever lived have been aquatic, because they need buoyancy to even begin to function.