[Ando55]

Lower body is a prime factor in gaining overall mass. The big 3 excercises: Benchpress, Squat and Deadlift are the biggest mass building excercises. I think the Squat and the Deadlift are way more effective in working more muscles and stabilizers when performing the excercise.

Quoted from Bodybuilding.com: "When it comes to adding upper body mass, you might be surprised to learn you can increase upper body size 10% by doing heavy leg work (squats). In fact, back in the early 80's when Tom Platz was trying so hard to bring his upper body to the same level as his legs, he actually began squatting more than usual upon hearing this. So if you aren't doing heavy squats, start now"

Many other sites and references support the claim that once you start to work on your lower body; your upper body follows up in overall mass. It's also important to work your lower body in order to keep symmetry and proportion. No one wants to have a great upper body with chicken legs. It looks just wrong once that is the case.