If you want to keep a smaller waist while you are gaining a bigger butt.
Then you need to tighten your transverse abdominal with core exercises,a long with weight training,healthy diet some sort of light to medium cardio to avoid too much fat gains.
Also, don’t overdo your bulk, add only like 150-200 cals extra ontop of your maintenance calorie intake.
For some people this can be hard to do, but it is worth a try;-)
I have now manage to gain some more backside and make my waist smaller,and I am not into eating too much now when I am trying to gain even more butt because I often end up gaining too much belly fat,and right now I want to gain slowly and keep my waist size as much as possible.
Abs exercises will not shrink your belly fat, but it will keep your belly strong and tight and you will look better too.
Have a nice Wednesday!