At barely over 4 lbs, my Sissy isn't as tiny as Tiffy - but she is very small in body size as well. Size was an issue for me in deciding on another Yorkie because I didn't want Sissy to get hurt with someone bigger than she is. She can usually hold her own, but it is a totally different story when you're talking meet-up vs. a permanent fixture
At 10 months of age, Angel is a little over an inch taller, two inches longer and almost a full pound heavier, though really skinny in comparison. Angel likes to play constantly and roughly and is constantly jumping on Sissy and biting her. Sissy gives it right back but will cry to be held when she has had enough and Angel is still rarin to go!!!! Angel is also a major jumper and has just started to take a flying leap off the couch (or bed) trying to jump on Sissy on the floor. Needless to say, this is NOT okay, so we are scolding whenever we see her setting it up. I do worry very much about Sissy being hurt still. I know Angel is stll a pup, but she plays and plays and plays, then plays some more and Sissy just doesn't want to be as active as Angel is.
When it comes to much larger dogs, Sissy usually hides. We have a good friend with a Sheltie who comes over frequently. Angel loves playing with her and occasionally gets kind-of pounded on by the Sheltie. Sissy does not playing with her and will either hide or want to be held.
Good luck with whatever you decide.