Another angle to consider is who weight trains in prison?
The guy in the article mentioned the "skinny guys" hurting their backs on the brick benches. What's skinny to a guy like that? I would be for sure, but I have a pretty high proportion of muscle mass for my frame size.
The point being that inmates who don't already have some size and are not capable of hefting some pretty sizeable weights (and perhaps those are the real skinny guys by a more reasonable definition) are not as likely to be found in a weight pile in prison. Not as a rule, but just in general. Simply stated, the weitght pile is the province of the big, or at least the strong guys. The exception, perhaps, being the guys who are provided some protection as part of a group.
Generally, in a survival situation, if your have a relative weakness, you might avoid those situations that display said weakness.
So, by that theory, the guys weight training in prison are big, because guys who weight train in prison are big. Get it?