Queenie is fairly large for a Chi., stocky, and generally very sturdily built. She is a classic "apple dome." Sophia is much smaller, very slender, a "deer-head" Chihuahua and appears to be part something else, perhaps small terrier. All of our dogs are rescues, so there is a lot about their pasts about which we do not know. When we got the dogs, Queenie was perhaps 2 years of age, and Sophia was was about a year old. We got them about 2 months apart (from Animal Control's "death row" ) The instigator of the fights is ALWAYS the smaller Sophia. I've had them both to the vets for check-up. The vet gave me no help at all re. this behavior.