My RB Cindy was exactly like that (she looked like Snowy too! ) She would challenge Rotties and other large dogs, and they'd always just walk away! LOL

Then my Grandmom had a Terrier that would attack Nicki EVERYTIME we saw her. Nicki is actually petrified of small dogs of any size, anywhere now because of it. She does LOVE my neighbor's Doxies though... so I guess it depends on certain conditions, like where or the dog itself. Poor dog is 100 pounds are scared to death of 20 pound dogs!