Why do small dogs attack bigger ones?

Good question. Probably a fence is your best bet.

Making a lot of assumptions but I'd say that the problem is actually with your Aunt and her relationship to her dog. She probably treats it like a little human so it thinks it's the pack leader and is instinctively defending its territory and its pack position. If your aunt was dominant over the chihuahua like you are with your pitbull it would know how to behave.

Lots of people love their dogs but do them no favours by not giving them a pack structure (with themselves at the top) to fit into.

your right it sbasically the problem with all the dogs.

i was walkin home today with my dog, and then a golden retriever type dog burst thru this jerryrigged fence, his head between me and my dog, hes in a frnzied state barking and my dog just grabs his head, and she shakes like 4 times good solid ones and im like jefita no and she stops and the other dog is still trying to bite.

it was crazy it was so unexpected. I just notcied she was alert , and she had t h "theres a dog around" look, and then BAM, RAWRAWRAWRAWRWA

The 'protecting' territory thing is a little different, our dogs bark at passing dogs and people, but when they're out of the yard they love meeting other dogs and never bark - even when passing yards with dogs barking at them. Strange beasts.