I'd say no, abs will not reduce your belly size, and infact they can add bulk to your stomach if you are building up muscles. It won't be a huge amount as no one gets abs that stick out 10 inches! But having said that, it can be very easy to do abs (sit ups etc) badly and push out your belly when you rise for example. This then creates a 'domed' effect which yes, will add inches after a while.
Best thing ab wise if you want to build muscle are transverse abs like the car pose, downward dog pose, plank and side planks and balancing exercises (standing on one food, close your eyes, then try that on the Bosu ball). The idea is that when you do these exercises you pull in but without sucking in your breath to do so. This trains your transverse abs and also helps your core.
For weight loss, its gotta be all over I'm afriad. If its any consolation I have always had a huge belly. I used to be properly anorexic (BMI 15) and STILL people would ask me if I was expecting as I just had such a pouted stomach. Its more then just fat, I think its a structural issue.
Cardio can help but most of all what you eat matters in regards to your weight.