Xenoblade Chronicles X, (22GB) Smash Bros 4, (15.7GB) and Tropical Freeze (11GB) are the only Nintendo-developed games on Wii U so far that would not have fit on a dual layered Wii disc.
Games like Mario Kart 8, Splatoon, 3D World, Pikmin 3, New Super Mario Bros U, Captain Toad, etc would each fit comfortably on a DVD-sized disc like the ones used by the Wii.
In fact, Wii games like Smash Bros Brawl and Metroid Other M are larger than most first party Wii U games.