I'm just trying to figure out why you want a big ass ...
Anyway, I guess (hopefully):
You are talking about more rounded buttocks (that is to say more specific and sexier), and not only (excuse me) the big butt. Are you sure you have really few (and if so, does that mean "flat" or just "less" compared to the rest of your body?
Assuming the two facts were true, I saw how flat butts ultimately turned into very decent shapes with heavy squats. This is what I would recommend.
If you only care about the size (not the shape), then yes, eat tons of carbohydrates, and since you accumulate more fat, some of them will inevitably be on your buttocks.
Remember, it all starts in the mind. Believe that any program you follow will give you the desired results. Pay no attention to the skeptics and the people who try to make you believe that they are all genes and nothing else.
Exercise types and steps to follow...
Step 1: fix the exercises you can see below
Step 2: good nutrition. Fitness is 80% of nutrition and 20% of exercises.
Step 3: adequate rest. To do this, you must think smartly and plan correctly.
Some of the best exercises for your buttocks are:
Squats - Normal, Wide, Deep (weighted and non-weighted)
Lunges - Static, Walking, Jumping (Weighted and non-weighted)
Hip Thrusts - weighted and non-weighted
Bulgarian Split Squats - non-weighted but performed slowlyand with proper form
Although these exercises are very effective for obtaining results, care must be taken not to over-train the gluteal muscles, otherwise the results will be opposite. Also focus on form first