Can you tell me if manual labor causes hands to enlarge by increasing muscle size or bone size or both?

Doubtful: Aside from some abnormal growth disorders bone stop growing in late teens early twenties in males and earlier in women. The muscles in the hand get stronger with use but not bigger (or piano players would have huge hands). With manual labor the body can respond to vibration and impact by increasing the fat and fibrous layers to thicken the palm and soft areas of the hand but not very much bigger.