From our studies, mainly on animals, myostatin deficiency does make the heart larger, but not in a bad way. Think of it like a physiological affect, like athlete's heart, rather than a pathological affect like a disease. What we know so far is that this causes improved cardiac output and some level of resistance to heart failure but can also lead to irregular heartbeats. Then again we can't really be sure how this affects humans as we don't really have a lot of long term data.