Genetics, growth hormone and diet thicken your waist, not weightlifting. The abdominal msucles are postural in nature much like the deep muscles of the back and legs, thus they are nearly all slow twitch fibers, that have little propensity for growth, perhaps 15%. This is the same reason a lot of people bitch about their calves, because the soleus muscle underneath the gastroc is almost exclusively slow twitch fibers, and depending on your genetic gifts from your parents, can wreak havoc in the development thereof.