Bikers generally are more tougher than the mob. Look up the history of the Hells Angels, Pagans, and other biker gangs, especially in relation to serving as muscle FOR the mob. Gangs like those make their members perform physical feats. You have to be able to lift a certain weight. You have to fight people at the drop of a hat. And not to mention, bikers carry just as many weapons. Bikers will stab somebody right in front of a cop. They don't care.
Mafia guys (for the most part) don't even work out. That's why they use bikers as muscle.

Bikers are like inmates, they do believe in some degree of working out, to whereas most mobsters wouldn't know what a weight was if it hit them in the head.