Eli used to have this problem... He seemed to favor little white dogs. I guess he's grown out of it... but really he hasn't been since October or so.. since he's so restricted these days.

Treo can bother some small dogs at the park... but usually she gets too noisy, the small dog yips/snaps and that's that. Treo seemed to miss the whole "you shouldn't walk all over other dogs" part of growing up. She's very clumsy and too physical most of the time. I tend to just let the other dogs put her in her place. If it gets too out of hand, then I will go get her and keep her leashed until the other dog has left.

Maybe you should look for a dog park that has separate sides for small and large dogs. I know one of the two here has that. Seems to work pretty well since the big dogs play so rough.