Your left trapezius is bigger than your right trapezius because you often shrug your shoulders. But the left arm and the right arm are thinner than the right arm, which is a common problem of most people. This is mainly determined by your dominant hand. If you are left-handed, your left arm will be thicker than your right arm. As for the left hand is not as smooth as the right hand in training, you should pay attention to your posture. Almost all movements will require you to sink your shoulders and relax during training. If you shrug your shoulders, it will inevitably increase the pressure on the trapezius muscle and reduce the stress on the target muscle, and the training effect will naturally not be ideal. You said that when practicing shoulders, the left side will appear sour before the right side, and the same is true. Your right hand should be the dominant hand. Then your left shoulder strength is worse than your right shoulder. Naturally, under the same pressure, your left shoulder will get tired first. If you improve your suggestions first, you can do it through unilateral training. For example, when flying dumbbells, you can do it with one hand, not both hands at the same time. This will make you more focused on the deltoid. Through a period of training, the strength gap between the two sides can also be well compensated.