It's a genetic mutation known as "double muscling" where myostatin, a protien that limits skeletal muscle growth, becomes inactive, and the animal produces excess skeletal muscle. It's most commonly observed on cattle. Many in the farming industry want to try and duplicate this mutation in their cattle because of the extra meat production, however there tends to be less fat in the meat and as such "double muscling" hurts the grade of the steak. Pretty much meaning that there would be no "prime beef" produced from a "double muscled" cow. As mentioned in the article, the animal still has normal sized organs and as such the extra bulk puts extra stress on the system and gives the animal a notably shorter life expectancy.

I find animals with this mutation to be incredibly interesting, however I would NEVER try to breed them as it is considered a health defect.