Generally, a large shoulder is a great trait. It will help create a longer, more swinging stride. (Which is then further aided or taken from by shoulder slope and angle, and from there the rest of the front limb)
Part of your guy's muscle mass over his shoulder may be that he is downhill in build. (I can't tell if he is very downhill or if he's just standing on sloped ground)
Now, a large shoulder can make correct saddle fit a bit challenging, since a properly fitted saddle should not interfere with a horse's shoulder, but larger shoulders need more space to rotate, which can force the decision whether to place the saddle too far back or on the scapula. When you have a large shoulder on a very short backed horse this can add even more challenge.