This photo is a good representation of their proportions in comparison to each other. 

I’ve noticed that, in fan art, they are often drawn quite unproportionate to their canon portrayals. And by this I don’t mean if it’s done in a particular art style! I mean they are deliberately drawn in that way to make one of the characters (usually Ben) appear much more submissive than he actually is in canon. 

(Personality-wise, he is not.)

I’d like to bring attention to the length and width of their limbs and torsos. 

First of all, they are different species. Keep that in mind. 

Rook is obviously taller than Ben because he is a Revonnahgander. That is also why his arms and legs are SO DAMN LONG COMPARED TO HIS TORSO, and the bone structure of his species is also probably different.  

BUT!! the width of their limbs doesn’t seem to be drastically different! 

It is questionable if Rook’s armor has anything to do with that. It almost makes me believe that in “No Honor Among Bros”, the substance Rook was under the control of had some sort of effect on his muscle mass to be more comparable to Fistrick.

So take what you will from this post, I thought it was a good reference.