The most common reason for large trapezia is overgrown upper trap muscles. Overactive upper trapezius muscles can often get bulked up and can cause stiffness in your back area. This leads to pain and prolonged feeling of discomfort in upper back.u003cbru003eIn order to slim down your trapezius, you need to stop over working your upper trap. This can be done by engaging lower trap more. One simple exercise is keeping your shoulder blades down and elbows tugged in your chest held high. Keep the shoulder blades squeezed together. This will engage your lower trap muscles and can help in slimming down your upper trap muscles.