I read an article saying GGG was 169 and some change the night of his fight with Danny. If I'm not mistaken, on fight night Golovkin weighed in 169.6lbs at the official 9am IBF second-day weigh-in. GGG doesn't pack on that much weight after the official weigh in for his fights. 180lb is definitely inflated for Golovkin's fight night weight. No way does he pack on 20 pounds. He doesn't even walk around at 180 in between fights. Which makes sense... GGG was 170 on fight night against Monroe who weighed 172 on fight night.
i think one key to GGG's power is that he might actually be left-handed (although he fights orthodox)--ie, he eats and writes his name with his left hand. no proof of this, it's just based on my own observations of watching GGG's fights over the past seven or so years, but consider: he's KO'ed fighters with both hands: his overhand right is his signature punch, but for instance, he KO'ed Macklin with a left to the rib cage. given the system and culture that GGG was trained in, i suspect that almost from the first day in the gym, if he or anyone else showed a southpaw stance, the trainers would just turn them around and if you look closely enough, GGG will sometimes adopt a southpaw stance after he throws a hard right, obviously with defense in mind, and once he recovers, he's back orthodox