I don't know how much this relates to the pot bellies but I find that with our pigs the larger ones are better at grazing. Bigger jaws, bigger teeth, bigger digestive tracts so they get more from the pastures and from hay without the need to feed any grain supplement to grow and keep condition. This makes it less expensive to feed them since the thing we have an abundance of is pasture.
I too had the concern when we started with pigs about them getting really big and handling big animals but they grow slowly on you so you have time to get used to them and they are tamed to us. Most of the breeders are only 300 to 600 lbs although some of the really big boars have gotten up to well over 1,000 lbs. Since pigs are so dense they don't seem that large.
-Walter