I know that we don't know what caused his heart attack, but just because he was born that way, doesn't mean he was healthy. Look at Andre the Giant. He was born huge, but his body couldn't take it. There are people born small and their bodies can't take it either.
The body's design was made to be a specific size within a range. MCD didn't seem like he was all that healthy. He was a very jolly person, but he seemed labored.
I'm just saying that if you're struggling to put on that extra muscle, maybe your body doesn't want it. That's all.