Which would you be more concerned with charging at you with teeth bared? A 20 lb Lhasa Apso or a 100 lb Pit Bull? With the Lhasa, a well placed kick would probably deter him from further aggression... with the Pit Bull, you'd draw back a stub for a leg.
Now, I'm just using a Pit Bull as an example, so all you pit bull owners, don't jump down my throat. I've seen 75 lb Golden Retrievers that looked like they wanted a piece of me. I almost lost a finger to a Labadoodle about 5 years ago that the owner said it was ok to pet and was supposedly very docile. A Chihuahua can be very aggressive, but can do little harm. Typically they will snap and bite very quickly...sure won't tear your leg off.
Please, don't get me wrong...I absolutely love dogs...all sizes and shapes.
The point here is that large breeds "can" be very dangerous under certain conditions. RV park owners can be held legally liable and insurance companies can, and will, put restrictive clauses in their policies on this very subject.
Ron