I have a large dog...but why I am scared of other dogs (small or large)?

thats natural, you are still insecure or your still scared of what will happen next, you'll have thoughts like "what if the dog is gonna chase me" or "What if the dog will come loose and bite me". you are already close to your dog and dont have any doubts of them doing bad thing to you so thats why you aren't afraid of your own.