Originating in 19th century Belgium by crossing local cattle with British cattle, the Belgian Blue cattle are gigantic bovine wonders. Their enormous size and muscle mass, sometimes called "double muscling," is a naturally occurring mutation of the gene for myostatin, a protein that regulates muscle growth. This mutation results in accelerated lean muscle growth.
I love when people don't read the article! Haha. It's not a disease, it's a genetic mutation. And in some cases, they're probably manipulating the genes on purpose to produce all those muscles. I don't think the meat would be tougher, since the muscles don't necessarily develop naturally.