Kiki is twice the size of Mazie, the cat is five times bigger than her, and the lab is....oh, probably 30 times bigger.
They all play and roughhouse together, and while everyonce in awhile someone might accidently get stepped on and yelp, for the most part the bigger animals just instinctively know they need to adjust to the smaller.
If you want to know the truth, I think Mazie inflicts the most pain.
They all seems to try to be so gentle with her, and give her her own way about things, because of her size....meanwhile, she's a dirty fighter, and gets some nips in.
I know it's a valid concern when you have BIG dogs and little dogs together (like my lab and the yorkies), but I think the concern over two YORKIES who are not the same size is a little exaggerated. I'd say as long as they are both under ten lbs, it's not a big issue.
PERSONALITY is a bigger issue than size, in my mind, but again, I think the vast majority of the time animals have good sense about how to play with one another.