I do know lots of small dogs that enjoy and are good at sports. Mainly cresteds, Paps, toy poodles. The problem I have the most is the treating process. With bigger dogs they're pretty much at waist level so tugging, giving treats, petting, etc is so easy. With a tiny dog that's pretty much on the ground even to pet the dog or to give a treat is quite cumbersome and not very convenient. That has nothing to do with their performance based on their size though. It's just I find the rewarding process difficult.