You need to do legs more often. As Nick said, do all compound movements and your full body in every session.
Voice of experience not good enough for you? I would listen to a guy heaps bigger and stronger than me. But if you like, let's have a look at
science instead.
"Exercises stressing multiple or large muscle groups have shown the greatest acute metabolic responses. For example, exercises such as the squat, leg press, leg extension, and bent-over row have been shown to elicit greater rates of oxygen consumption than exercises such as the behind-the-neck shoulder press, bench press, upright row, and arm curl. In addition, these exercises have elicited the greatest acute hormonal responses. Deadlifts, squat jumps, and Olympic lifts have produced greater acute 22-kDa growth hormone and testosterone responses compared with exercises such as the bench press and seated shoulder press.
"[...] We have shown the importance of the total-body resistance training (as opposed to upper-body-only training) in women [and men] for improving overall muscular strength, hypertrophy, power, and physical performance.
"[...] Considering that multiple-joint exercises have been shown to increase muscular strength, hypertrophy, and power, maximizing performance of these exercises may be necessary for optimal gains."
Put in plain English, big compound lifts make you bigger and stronger the fastest, and doing squats, leg press and the like will actually help to make your chest and arms bigger.
