You have to remember that Yorkie spelled backwards is Rottweiler!! At least to them it is. Usually, they play pretty rough with each other before they are placed in their forever homes. So they just "assume" that other dogs are going to play with them just like their siblings did. They don't understand that the other dog is 25 pounds bigger. They just wanna play. Just make sure the bigger guy doesn't get too rough and they'll be fine. Pretty soon, the Yorkie will have the other one trained.
Suz