I have been attacked by large dogs and I know they can be a real problem when allowed to run loose.
I have been walking for 18 years. I have came across both small and large dogs. Never has one of the large dogs even threatened me. The small dogs, well that's why I carry a stick. I have been pinned in the ditch by 30 lb. dog and danced the jig fighting off a 10 lb. dog who wouldn't even back off, just kept trying to bite me. Another 25 lb. dog ran circles around me. Although they were smaller, it is still very frightening. Ironically, the 25 lb. dog has a 35lb. sibling that is a pit bull 'type' dog and he is friendly! I agree, large dogs could hurt someone worse than a small dog, but my problems have been with the small dogs.
I'm NOT dissing small dogs in any way, I love all dogs, just sharing what I have dealt with.

*dogs weights are approximate*