There are many weight trainers at any gym that tend to focus almost entirely on building up their upper body muscles and neglecting leg workouts for the most part. They may do some training for their legs, but don't lift near as hard as they do for the upper body. This is what causes the cartoonish, Johnny Bravo look of having huge upper body muscles and average legs.  If you want optimal overall muscularity you need to focus on performing leg workouts with the same amount of intensity as you do for upper body workouts. If you do quadricep exercises that include squats you'll likely even add more size to your upper body since squatting involves so many different muscles and gives you a boost in your testosterone levels.