Nope, we've seen dogs of every shape and size use the Hub, from 6 lb Chihuahuas to 160 lb Boxadors. We've even had cats and pigs use it! The touchpads are triggered using capacitive sensors, so they are not weight dependent. Even a small dog has the ability to trigger a CleverPet touchpad.
Check out Ripley the mixed breed and Daphne the Great Dane, both excellent Hubbers!