One of the things that happen with smaller/little dogs - especially that literally fit in someone's hand or in a bag - is that they are often not allowed to "be dogs" and their behavior is considered "cute" or funny. Many times they are constantly carried around, not exposed to things other dogs are - and because<especially in the case of the chihauhas in handbags - how many times have I seen that?> people reach for them - and dogs often don't like people reaching FOR them - and the small dog finds that actually very scary and so they respond with defensive aggression - fearful. The outreached hand or person retreats - so dog learns - I act "big and scary, hand goes away". That cycle is repeated and enlarged upon over the course of time.
and children are at much more risk of getting bitten in the face, which is usually a severe injury due to location. Kids do many things naturally when being introduced to dogs that put them in this dangerous position - besides just being at the same level by chance, usually - kids also tend to stare -rude- some dogs tolerate it but most dogs, even trained therapy dogs, do not appreciate being stared at <therapy dogs have just learned that it's a weird "person-thing" and let it go at that>; they approach head-on and fast <also rude and very scary for many dogs>, they make erratic movements and noises and they often pat on top of the hand and right around the dog's face - also very rude.
That's why I really appreciate it when parents have taught kids the right away to get to know a new dog.
I work with many wonderful bullies in the shelter and in rescue, as well as people's personal dogs.