I see Brook as 1.5 weight classes lower than Golovkin. I see Pac as 1.5 weight classes lower than Mayweather.
Any recent shirtless pics of brook? We need to see how lean he is at 160, GGG has the bigger frame for sure, kell has a smaller frame but might actually have more muscle mass than GGG, GGG's larger bone structure, head etc account for a fair few lbs, where as brooks smaller lighter frame enables him to carry more muscle mass for the same total body weight. Not that muscle mass is the be all by any means but brook is very muscular and i don't think the weight jump will be as much of a problem for him as it would be for khan who is not very muscular and at 155 was soft, most of the added weight was bodyfat not muscle.
I think it's a fair bet that Brook has a little something in his shoes to reduce the height difference. Pacquiao did this against Mayweather and Klitschko against Fury. In each case the true height difference is around 2-2.5 inches and the shorter man has managed to make it look maybe an inch.
There used to be posters of fighters with measurements of neck , bicep , wrist , chest circumference . Seems like that isn't done anymore for some reason . Hopefully Kell last a few rounds , who knows what will happen ..... all it takes is one punch