[dottiesyrky]

I think small dogs are scared of big ones when they lunge forward in play. My dog does not like that either.
You suggest that she is ok when on the lead, so perhaps she feels safer knowing you are in control of her. Perhaps if there are larger dogs around you could keep her on the lead and take her slowly to the larger dog until she gets scared, and then lead her away to safety. ( in her mind!). If you cannot get her over the big dog thing, she may just not like them!! In that case you might only be able to let her lead free in a safer place, free of big dogs.
I keep my Yorkie on a lead always when outside as I never know how other dogs may behave with him. In enclosed dog parks here, where dogs can be lead free, they usually separate big from small dogs. I do not like dog parks as I am scared my dog may pick up diseases or fleas!! But that is just my choice.