It has to do with genetics. Brian Buchanan is a genetic freak! Sergio Oliver had the same challenge in power lifting,narrow hips and waist is a disadvantage in power lifting BUT they have a BIG advantage in bodybuilding! Work with your genetics best you can! I'd personally rather have smaller hips and waist.
First make sure your diet in down pat. It can take a long time to diet down. Ive been doing this Five years and keep learning.
Next Training.Vince Gironda teachings are against back squats saying they make your waist and hips expand. Chris Cormier eliminates Squats a few too a couple months before his competeions, beleiving this same thing; although he does do them offseason(not close to a contest)
here is a solution if you have this challenge
If your waist is too big ,with diet being the best you can possibly get it, eliminate them.
This is me personally. I have wide lats and decent quad size that my waist is 37 and actually my body looks bigger because of the exaggerated proportions so I can get away with doing squats for a while. Now when it comes cutting time Ill just do front squat, leg press, hacks or smythe. I don't know if barbell squats widen the hips or not. they do widen the waist more than say A smythe or hack squat.