You likely won't want the tri-top with a shape like you describe. All binders other than the 988 have rolling-up issues, and they roll worse on people whose bodies are curving outward at the bottom of the binder.
You're right that they all fit pretty slim, but the cotton and nylon they use are extremely soft and stretchy. The only semi-rigid binding material is the powernet (found in the top part of the 988, as well as throughout the 997 and 983 and on the inside layer of the 998). The bottom part of the 988, like the other non-Powernet binders, will pretty much stretch to fit no matter how big you are, within reason. I could fit 2 of me inside my extra-small Cotton Concealer without exceeding the fabric's stretch capability. And I'm not that tiny.