The most effective way to reduce the apparent size of one's shoulders is through corrective stretching and exercises. Shoulders can appear large or hunched due to bad posture and muscular issues, which cause them to be raised and rounded.
Two common shoulder problems are upper-cross syndrome and having winged scapulae. The first is a postural condition wherein the upper body curves forward and the shoulders lift upward, creating an un-aesthetic look with prominent shoulders. Winged scapulae result from anterior muscle tension and weak serratus muscles, causing the shoulder blades to "wing" out and create a bad profile.
Exercises like pullups, pullovers, rows and high shoulder pulls can all strengthen the back and shoulder muscles, pulling them down into a proper resting position. Corrective stretches, like shoulder dislocations, and certain yoga poses can help increase flexibility and also allow the shoulders to rest in a more natural position.
As always, a person looking to improve shoulder appearance should talk to a doctor before starting a workout regimen. Depending on the level of fitness and joint mobility, certain exercises might not be healthy. Some things, like yoga or tai chi, are safe to try so long as the person doesn't overwork or injure himself in the process.