Just checked my models. The Outrider horses are a little thinner than the chaos horses. The outriders also have a kind of "upside down V" shape to their legs, which lets them just sit on the saddles, while the marauders are more semi-circular, and pop on and off. I imagine that you could fit an outrider onto a chaos mount, but it would take some working with a modelling knife to make it fit properly.
Hope this helps!