Might I suggest: The mighty marvel, the $4.95 AB Wheel at Wal-Mart, Target, etc. Or the double wheel model, under $20 bucks ( or yet still, the mag dual wheeled, 5 forward gear positioned, semi water cooled rear end axle compound drive ratio meshed to tranny..for all you guy's who think something special and complex is needed to work the ab's...for the people with less confidence, there is the the newer model with training wheel on it).
The Ab wheel trains the whole ab wall as well as the obliques. Advance guy's start at a standing jack knife position. Try it on a slight decline or with feet raised on a bench, etc.. Might try twice a week, of 15 to 30 reps of 2 to 3 sets, slowly back and forth.
The ab's are not a thick muscles group by nature. But as a short ROM muscle group, very powerful. Strong abs help the lower back when doing squats, DL's, etc. Avoiding injury along the way. Like wearing a built in lifting belt. If you train the ab's heavy, like 100lbs+ for reps of 5 to 9 (yes, a lot of very powerful men include lower rep training for ab's), than "muscular" thickness can occur. Seen some guy's who could just about hide a dime between the ridges of their ab's.
Gironda stress the importance of the Solar Plexus (which some call the second brain...great influence on the CNS, metabolism, digestion, etc) and why too much direct ad work could slow or halt gaining muscle. Not to mention causing nervous system injury. Might check out his version on ab training.
Coleman has a big belly, not so much from ab development or being bloated, but because his inner organs have grown too large in size. Those chemical goodie's he took for years not only make the muscle structure grow, but also the inner organs & heart. The man must be going though painful day's now as he gets older. Got to pity him, in a way. Good Luck.