Leah, Big's weight is probably the reason he isn't a jumper in spite of his adequate hindquarters. My Abby (rainbow bridge) was obese, and she wasn't a jumper either although she was a Manx. The only thing that would make her try to jump on anything would be FOOD. I once saw her try to jump on the bed and fall off, dragging the covers with her!
Tuxluvr, I never thought that Manx looked like souped up sportscars until you mentioned it, and I think you're right!!LOL!! Manx have the nicest personalities, very sweet and companionable.