The maltese standard calls for 4-6 lbs preferred
so the standard size is very similar to the yorkie's size.
But, yes, I have a yorkie who is 11 lbs and a maltese who is just under 4 lbs, and they get along great. There are absolutely no problems with their sizes. They don't hurt each other, but my yorkie is very docile. He's not rough, but the little 4lber is
So, she's the one that pushes him around. LOL
We take them both out with us in the stroller. She looks so tiny next to him, but like I said, there aren't any problems with the difference in size with my two furbabies. If we take one out with us, the other goes two, so they both go places with us, and they keep each other company. They absolutely love each other. I'll try to post a picture later on.