Little dogs seem to be meaner too. Its like they're over-compensating or something. I've been around a lot of dogs and seen a lot of people bit for different reasons, usually having to do with playing too rough, getting in the way of dogs fighting, accidently startling a dog, things that don't have to do with exact aggression towards the person. The ONLY time I saw clear evidence of that is when my little brother was just 5, we were standing outside the neighbor's house talking to her, and suddenly her little miniature poodle came FLYING out of the house, went straight for my little bro and attacked him! And bit him HARD on the leg! The poor kid was terrified of dogs for the longest time. We got him over it by getting a new puppy that was "his" - worked like a charm!
My big dog Rosie seems to have a problem with little dogs too. Not scared of them, just doesn't seem to like them. We are currently sharing our home with a Westie Terrier that we have to keep an eye on Rosie with because she sometimes acts aggressive with him in a way she doesn't with larger dogs. I think that with terriers and poodles in particular, they don't tend to act much like larger, more pack oriented dogs do. They stare and don't avert their eyes, they don't use as clear body language. Perhaps its because they've been bred so much for other characteristic s its put them kind of far away from their basic instincts. The Westie doesn't even obey pack order as clearly as the other dogs do, with the other dogs and with the humans. Sometimes I think Rosie's aggresiveness is because she it trying to remind the Westie of his place. Other than that, Rosie doesn't even really interact much with the Westie. She kind of acts like she doesn't even think he's a dog at all! Maybe there's a little something to that - because of their different behavior, they might be seen as a bit foreign, so maybe that has something to do with your dogs' fear of the Yorkie. Dunno, just a thot...