Since you mention that you’ll be on a bodybuilding stage, I’m assuming you are aware of the strict diet and other training protocols associated with competing in an event of that nature, so I’ll focus my answer on the specific area of abdominal training.
As with muscle fibers in other areas of the body, abdominal fibers will respond to a program of regular resistance training by increasing in thickness, as long as the exercise stimulus is adequate. To provoke hypertrophy (increased muscle mass), muscles must experience a load that they are unaccustomed to and they must experience that load for a certain duration (known as “time under tension”). That is, muscle fibers that are engaged in between 30 to 90 seconds of time under tension per set over several workout sessions (e.g., 2 to 4 workouts per week over several months) tend to respond by increasing in thickness. In other words, the variables of time under tension and load (weight) are manipulated through exercise program designs that call for 12–15 repetitions (which typically take between 30 to 60 seconds to complete) for 4–6 sets at a weight that causes muscle fatigue by the end of each set.
Be sure to add an appropriate amount of weight to abdominal exercises, if hypertrophy is your goal. Most exercisers train their abdominals using only their bodyweight (e.g., crunches and planks) and neglect the aspect of increasing load through progressively adding weight to stimulate muscle growth. Holding onto a plate weight or dumbbell while doing decline-bench crunches or performing kneeling spinal flexion while using a cable machine are ways to increase the load on the abs.
The other thing to consider is the fiber-type composition of the abdominals. While individuals vary, most people have a higher percentage of slow-twitch fibers (rather than fast-twitch fibers) in the abdomen. While both types of fibers can increase in thickness, fast-twitch fibers tend to hypertrophy more so than the others. In other words, genetics play a big role in the ability to develop thick abs. You can read more about fiber-type differences of the torso muscles in this previous post. You’ll also find information about how frequently you should be training your abs in the link below.