Well, If you want to know the reason why this happens (and why big dogs are generally more friendly than little dogs), it's because people with big dogs are forced to train their dogs more and do socialize them more because they are big and can be menacing then a smaller dog. Im not saying that is always the case there can be mean, big dogs and nice, small dogs. I have just seen the general difference between the two, many times.
Why would you push a dog?
Hey c ;)