The size only reflects the amount of data, images, video, sound, and other resources needed for the game. Many of the Guitar Heros/Rock Bands weigh in around 4 GB due to having 3GB+ of music and charts along with probably a gig of textures and lighting. On the flipside, Wii Sports uses basic textures and only a few sounds, and fits inside 300MB.
Brawl is a classic case; it contains more than 2GB of full-motion prerendered video.
It is interesting to note that Twilight Princess, which is a port of the GCN version, is barely 1 GB in size.