I probably wouldn't get caught up too much on the muscle part of the equation. There was another study done that concluded that intense resistance training floods the body with all sorts of cancer fighting chemicals. So the increased muscle mass and strength are really just by-products of the regular resistance training. But to answer your question, I think that increased muscle mass only becomes unhealthy when the ability of the muscle to contract exceeds the ability of the surrounding tendons and ligaments to support it, which results in tearing something that requires surgery. That's why I'm not really a big fan of 1RM training.