I have 2 Yorkies. One just hit 4lbs, and the other one who is half Silky Terrier is 5 1/2lbs. They are little rockets, and they climb up on the top of the couch, and dive bomb off after each other. Just today Lucy was trying to jump up to me in this wooden chair we have, and she bonked her head on it (HARD) and I was worried, but she just looked around, shook her head, and proceded to licking me all over. lol!
They're fairly hardy, but IMO, I think their Diet has alot to do with it, as well as how well they are bred (a well bred dog will have more of a chance of being less fragile than a puppymill or irresponsibly bred puppy IMO). My cousin had a Toy Fox Terrier (I'm gonna say she was around 4lbs) who jumped from my cousin's arms and broke her leg. My dogs, when being held, will climb up the other side of my arm, and jump to the ground from my shoulder, and think nothing of it. I don't know what could've been different, in Sweetie (the TFT) and my girls, but it must've been something. My girls are fed Nutro Natural Choice, and I think it has to do alot with how hardy they are. Maybe I'm wrong. In general, I'd say toy breeds are fragile, but it would really depend on what you're comparing them too. You have to be considerate of their size, definately. My 12yr old brother has stepped on our girls several times. They're always by your feet, and you can be walking, or running, and they'll get tangled between your feet, and trip you. Imagine if you fell on one! :yikes: There are precautions that should be taken with smaller dogs, but I think it's worth it.
I love my Yorkies too much!