It's a 2D game, and theres only a small arena to fight on, yet it's bigger than any other Wii game, including CoD, which has a huge campaign, lots of models and textures, and much more...
Try completing 100% of the game and you'll find out. There are tons of contents in form of videos, music, trophies, play modes, npc, events, easter eggs and etcetera. You just didn't explore the game enough but if you try you'll see that there's like an infinite amount of content.