It's bigger for several reasons, one of which is the bundled iLife apps. GarageBand and iDVD support files can take up close to 4-5 GB between them. Printer drivers can also be close to 2 GB.
I only trim my install on iBook Dual USB, as it only has a 10 GB hard drive. I've got the install down around 3.5 GB, which is fine with me.