Dogs are generally comfortable and relaxed off leash. Large dogs are also comfortable and relaxed on leash. But small dogs... not comfortable on leash! Because of that leash, they know that they cannot run away if a big aggressive dog comes at them. So they yip yip yip in panic and fear, always nervous, hoping all that barking will scare away any big dog that might come after them... even when there aren't any big dogs coming after them!

I have a new nick name for such a dog in such a situation: A "European" dog.