@PyreneanAubrie he's about 4 inches tall. If he was any bigger he would be terrifying, although, had he been bigger I imagine DD would have asked me for help training him sooner, or at least mentioned that he hates everything and everyone that is not part of his immediate family.
Like most small dog owners she didn't take his aggression seriously because she could pick him up and put him in her coat when he started making a spectacle of himself, which did nothing more than reinforce to him that all of these strange things were a threat and needed immediate action to keep them at bay.
To be fair, despite working with and studying dogs and their behaviour for years, I am guilty of the same. When the big dog showed reactivity around high-value treats we didn't find it cute or amusing, we took immediate action. When one of the chihuahuas did it we laughed at her, recorded it, and then worked with her for her own sake. It's funny when a chihuahua does (it's really not) but when a rottweiler does it, it's an issue.
Training bigger dogs is also easier because people are more sensible around them. If a strange rottweiler was growling and snarling I imagine people would cross the street. When a tiny ball of fluff is doing it people laugh and try to touch him anyway
I have the confidence to tell people back the fuck off away from my dog. DD didn't when she was out with him which also made it worse.
This is why you find a lot of small breeds have issues with aggression whereas bigger dogs tend to be more gentle and better trained.
Here's the culprit looking all sweet and innocent whilst secretly plotting a mass killing spree.