Weight training can hamper your playing. How? Two ways: poor form can cause injury to soft tissues, joints, neck, and spine. Training with heavy weight, even with perfect form, shortens the muscles (that is the principal manner in which they bulk up), which stretches the tendons on both ends of the muscle, putting them on a higher baseline of stress. Stressed tendons damage more easily, and as tendonitis is a common injury among cellists, we need to engage in this activity with some caution.
Do I mean by this that you should not train with weights, absolutely not! The question becomes how should you train. You need to train so that your muscle system is balanced, as unbalanced structures are prone to injury. Weight lifting does not strengthen the muscles of the shoulder capsule (the 4 muscles of the rotator cuff) and building mass all around them makes them more vulnerable. Strengthening the rotator cuff is difficult to do because of its structure, and it really takes expertise that you won’t find in any gym to do this. Absent this expert guidance, the safest thing to do is use lower amounts of weight with higher repetition and great attention to both form and variety. Don’t rely on a gym rat trainer to give you help, most of them I see in gyms are not competent to protect you from injury. Their interest in too many cases is to bulk you up, which is not our best goal.