In the above UFC 95 video, UFC President Dana White asks the question at the center of the main event: "The question is, Is Diego Sanchez going to be too big and too strong for Joe Stevenson?"
White also notes in the video, "Diego's good at overpowering people. He's good at getting the takedown, getting on top of people and grounding and pounding them."
I think the strength and size advantage that Sanchez has, and the way he'll use that strength and size advantage on the ground, is why he has to be considered an overwhelming favorite.