this is an issue that we all should be concerned with in the sport of BB. a large aspect that will determine how your waist looks is genetics. this is something that can't be helped. the size of your hips can't be changed, no matter what you do but we all have weaknesses and the key to building an outstanding physique is learning how to create the best illusion to hide our weak areas while bringing out our dominant traits. without seeing you, its hard to say exactly what the issue is, but often time, guys with shorter midsections have the appearance of a wider waist. these same people often have lower attaching lats, so with this downfall also may come with a blessing when you turn around and hit your back poses. 

first i would keep the midsection in check year round. it takes tons of food to grow new muscle, but this food can distend your stomach. make sure you consume plenty of fiber to keep the food moving through your system. on top of green veggies, i would use a daily sugar free fiber supp. this, in itself, can make a big differecne. it becomes more important during the contest season, when you're getting less fibrous carbs. watch the way different food sources effect you. if you're eating lots of steak and your gut seems to be popping out more than before, add in some ligher foods like white fish, that tend to be easier on digestion. 

watch your obliques. as they thicken up, your waist becomes more blocky. avoid twisting movements when working your abs and when doing movements like DB rows. on the note of abs, use higher rep bodyweight crunches vs heavy weighted crunch movements

and remember, the wider your shoulders can look and the bigger your legs are, the smaller your waist will appear. you can only genetically get your waist so small, but you can create an illusion by comparison. sadly, heavy squats and deadlifts can thicken the waist. i would never say that you should remove these movements from your program, but i've found that moving to more of a moderate weight with squats for higher reps works well for quads while not activating my more core as intensely

finally, insulin is your enemy when it comes to a small waist. keep the fast acting carbs under control. use them when you need them but be aware and try to keep them low at other times of the day. if you're using exogenous insulin, you may need to cut it back. exogenous insulin can be a good tool for some, but it is also destroying a lot of great physiques now days, IMO.