I see overprotection by dog owners frequently at our dog park also. One owner I see frequently always takes his dog out of the park whenever my rescue dog and I enter. It makes me mad because his dog is one of the few my little dog wants to play with.
I have two small dogs and am extremely cautious with one of the dogs because some times you will get an extremely large dog that just wants to play but does not no how to play with smaller dogs generally this happens with pups that only 3-6 month earlier would of been playing hard with the smaller dog and there would of been no problem because there would of not been a big size difference. give that dog a few more months and some time around smaller dogs and it realizes it cant play the same with the smaller dog.