Alot of it is genetics. Bone structure. However, with your hips (which are very nice in your avatar) if you just focus on diet. Low fat less than 30 grams a day, low carb less than 20 grams, and get in 1-1.5 grams of protein per pound of bodyweight and some light cardio (walk for 45-60 minutes on an empty stomach every morning) then your abdomen will shrink by more than several inches in just 2-3 months. This is basically a Ketogenic diet if you wanna research it.
I'm a competitive bodybuilder and a judge, so trust me, for what you are trying to accomplish is very attainable with your structure.