Situations like this call for killing them with kindness. The last thing you want is strained relations with neighbors.
Get your dog and the neighbors big dog together with both owners and get them used to each other. Once they know each other the big dog won't have any reason to come over to your yard.
When you present the idea to the neighbor say something like "Hi, i notice your dog keeps jumping my fence and i was a bit worried since my dog is so small. Would you be able to spend a couple of afternoons with me in the yard so our dogs could get used to each other?"
Only good things will come from this. Your dog makes new friends, they put their dog on a leash, you make new friends - or all of the above
good luck