To me, it just seems that Golovkin possesses a much larger bone structure to that of Stevens which in turn provides an optical illusion of Golovkin being the larger man (at least in my eyes) regardless of them being the same weight.
He walks and fights 168-170 apparently, but obviously comes down a bit for the weigh in. That's a fairly normal fight weight for a MW - though plenty fight bigger (JCC jr would get upto 180+ or so by accounts), just a lotta guys bulk up more between fights.
What's wrong with Perez vs Mago? That has to be a fun fight; whatever else you say about him, Mago always comes to fight, and they've both still got the zero. What's not to like - unbeaten boxer v's brawler HW match-up?