That's Karen Pryor (Don't Shoot the Dog), another fabulous trainer and pioneer. She also has a website: clickertraining.com

It doesn't matter how friendly a large dog is towards a small dog. They could adore the dog they're playing with, but the fact is that dogs are rough when they play. And when large dogs are rough with small dogs, the latter usually loses.

I have two large, very high-prey driven dogs, Ivy and Giselle, and a small little Pekingese. Giselle spent the first 3 years of her life making a living off chasing small, white things. Yet, my three pups live in harmony *at home*. In a dog park, however, all bets are off. I think physical separation is just best. If your dog park doesn't have separate areas for small and large dogs, ask/petition for them. At our local dog park, if a small dog comes into the large dog area, we kindly ask them to leave and go to the small dog area. Better safe than sorry!