Again, like I said, "indeterminant growth. Average wels is not over 16.5 feet, and your average Pangasianodon gigas does not get to be as big either. Its like saying that asians are the tallest people in the world because of Yao Ming.
Arapaima is another fish that you can throw in the list of fish contending for "largest fw fish in the world".
Mekong Giant Ray Dasyatis laosensis, and the white sturgeon Ascipenser transmontanus (to 20' * Peterson: Field Guide to Freshwater Fishes) might also have something to say about that.