Just because a dog is small, doesn't mean it's not powerful. Sled dogs are usually smaller than you think they are. Any dog (outside of the Toy and Teacup variety) can do this activity, so long as they like to pull.
I've heard of people doing this activity; there's a small group that do it on mountain bikes around the Bend area. And, I saw something on Animal Planet about this activity as well.
Besides, dogs have problems with obesity as much as their human counterparts do.