Many dogs of all sizes are leash reactive. What may come off as aggression (growling, lunging, etc.) is your dog’s way of expressing his frustration of not being able to do what HE wants to do because of the leash. Some dogs’ leash reactivity is also fear driven. You need to teach your dog that the other canines should not be feared. With my pet, when we see another dog, I will stop and make her sit or lay down. I talk to her calmly and feed her some high value treats. That way she associates something positive (treats and calmness) with the dog. Granted, it doesn’t work every time, but she is getting better. After a few minutes, we walk on. Her reaction is a lot less “severe” as it could have been. She is very ball-driven, so there are times when I switch it up and take a tennis ball with me. When we see a dog, I show her the ball and her focus goes directly to me and the ball. Having the dog focus on you is so important too. Your dog needs to look to you for that feeling of safety and security.