A deficiency in myostatin can and does occur in humans. Take, for example, a German boy who could lift 7-lb. weights with arms extended when he was 5 years old. You can find an image of him when he was a few months old in the article. Since his identity was not revealed, current images are likely hard to find. (Also, I don't want to be put on another watch list for trying to find them...)
Researchers have expressed health concerns in the boy's future due to his increased muscle mass. (Warning: paywall ahead) Research shows that a reduction in myostatin may also cause an increase in bone formation. Additionally, animal breeds with a myostatin deficiency have reproduction, dietary, and care issues relating to their increased muscle mass.
Humanoids with a myostatin deficiency would be bulkier through increased muscle and bone mass, but I would also anticipate them being fewer in number, due to all the special concerns that apply.
It's important to note that most breeds with a myostatin deficiency mutation have been engineered or selectively bred specifically for that trait, such as the Belgian Blue. This may be because nature cannot normally support such a creature due to the health concerns, especially as they apply towards reproduction.