It's called weight cutting. Golovkin is a natural MW and Jacobs obviously dries out to 160 and fills out by the fight. It's a widespread concept in boxing as well as MMA. That's why we must take a lot of these multi-division champion fighters with a grain of salt. They don't move up in weight, they just cut less.
Notice his overly aggressive demeanor during the intros and fight? And miraculously staying conscious?!
