We have a "Bigger" dog as well in addition to our Yorkie. He weighs about 50lbs I guess. He is a German Sheppard mix. We got him from the shelter and he is the best dog ever! He was first in our home. Rusty (our yorkie) came in years later. Sometimes Stedman gets very excited and starts playing to rough and I just have to say No and he will stop or I will separate them. Usually though it is Rusty beating up on Stedman, climbing all over him, chewing his ears, licking his face, etc etc. Every once in a while Rusty gets stepped on in mid play and he will let out a little yelp but then he is back to attacking Stedman with full force. Stedman is very gently and docile though and pretty much lets Rusty do what he wants to him. He is older though (7) and is pretty much in the lazy stage of his life.
We like to say Rusty is a German Sheppard on the inside and Stedman is a yorkie
Good luck with whatever you choose to do. Sometimes time is the only thing you need to make it work.