The 757/A320 size seems to be a sweat spot. As you move bigger, or smaller, the cost per seat-mile increases. Newer designs are better than older designs, obviously, so a newer design that's farther from the optimum size will likely do better than an older one that's closer.
I don't know how well the newer technology in the A380 does at overcoming it's very large size. It may do quite well. The 787 should be an efficiency leader when it enters the market as well.