The general proportions paint the image of a generalist rather than a specialist, and this is consistent with the known bite marks that support feeding on a wide variety of diplodocid and macronarian sauropods, as well as thyreophorans. Clearly this served it well, since Allosaurus was by far the most common and successful theropod of its ecosystem.
Almost 70% of all dinosaur fossils in some Morrison localities pertain to Allosaurus. And not all of that can be ascribed to being predator traps, because it also far outnumbers all other contemporary predators in these lagerstätten. In Cleveland Lloyd quarry, a minimim of 46 Allosaurus were recorded, but only a single individual of Torvosaurus and Ceratosaurus respectively (Gates 2005).
For a large-bodied theropod, its skull and teeth are only moderately sized and the forelimbs are comparatively long and very powerful. The light skull, compact body and powerful legs suggest a high level of agility for an animal of such size (averaging between 8 and 9m, some specimens exceeded 11m, and possible relatives based on footprints could have significantly exceeded 13m).
Why was it so successful? The answer is probably hidden in its jaw and neck, which are uniquely adapted for both quick and powerful neck-driven strikes, with a lightweight skull able to withstand tremendous forces despite a weak bite force and wide gape, and a neck adapted for both rapid and forceful cranial depression and retraction, while remaining very flexible and capable of quick movements. In short, an extremely powerful yet versatile weapon, equally suited for pushing and tearing deeply into the flesh of a sauropod, shredding the neck of a stegosaur, or striking at a small ornithopod.
Based on the humerus of DINO 2560, the Saurophaganax OMNH 1935 would have measured 11.3 m, for the largest specimen, 13 m is no longer a credible figure.
it is 8.8 m and it has a humerus measuring 42.5 cm according to Scott Hartman, scaled isometrically it gives you an estimate of 11.28 m or less.
Maybe Saurophaganax had relatively shorter arms, but that still gives you a range of 11-12 m, rivaling AMNH 5767.