I didn't see any sort of a pattern about the size, but to be fair he wasn't my dog so I didn't see him interacting with other dogs most of the time. He was fine with both greyhounds the same size as him and whippets that were much smaller, so I don't think it was about size

That said you have reminded me of an entertaining memory. I worked at a greyhound rescue centre for a little bit (not the one my aunt's dog came from), and I was just about the only person there that wasn't a functionally-retired little old lady. One morning while we were walking the dogs, a squirrel darted across the path. It was carnage. Grannies scattered like bowling pins. Everyone was okay