I'm having the same problem with Bitsy and my daughters mini doxie. Bitsy is 5 lbs and Elwood is 10 lbs., and Bitsy is the main one that starts it all
She chases him, growling and snipping at him. He never growls back though, I guess he's just putting up with her. Thing is, this happens every day! It is driving me crazy
She has done this since day one! And we had Elwood two months before Bitsy came along.