If you want or have any desire to gain more mass and size you MUST work the entire body! Yes, that means hit the legs! Don't skip squats no matter how tedious they are to you! Having a balanced body is keen to seeing more muscle growth and mass difference. Working legs along with upper body will allow your body to produce more testosterone which will help you see quicker and bigger gains in your workout! And quite frankly who cares if you can bench over 300 pounds if you can't squat more than your bench! Now get to it!