What the fuck is with this "winning" rhetoric in this thread?
There are too many people weighing in who have no business owning a dog. People like to talk shit about small dogs because it makes them feel tough but they're usually the very dog owners who fail to read body language and take preventative measures in iffy situations.
The reality is, all dogs require a certain amount of personal space, just like people. And just like with people, not all dogs pick up on cues or care to show basic respect. In dog world, that can lead to a fight, as is what surely happened here. The woman shouted that the Pitbull came around the corner and attacked. I could easily see this being the case: the Pit came around the corner of an aisle, got into the small dog's face (by accident or on purpose), small dog gave a warning snap, and the larger dog escalated. Being larger makes escalation far too easy, not just in dog world either.
Small dogs especially dislike when someone hovers over them. It can be considered threatening behavior regardless of size but small dogs are even more fearful when it happens and yes, even though it doesn't make sense to us, snapping is one of the ways they try to deal with it. Larger dogs who don't respect personal space can be harder for them to cope with. When I got my Chihuahua, my Lab mix went right up to her and started sniffing. She gave a warning bark/snap so he stopped, he didn't escalate into an attack like what happened here. When I introduced her to our second dog, same size, Terrier mix, he immediately knew - despite not having seen her snap before - not to stick his nose up her ass. He sat a respectable distance away and watched her from the corner of his eye so she didn't misread his body language. To this day, he's still her favorite.
People want to make it seem like this dog got all tough and picked a fight but it's far more likely that it got spooked. And as both dogs were on leashes, no one was really being irresponsible. But I will say this, I don't trust larger dogs around my Chihuahua, they frequently don't respect personal space so when I've got her out for a walk, if I see a dog bigger than ~15 pounds coming our way, I immediately pick her up for her own comfort.