Did any of the big dogs show any signs of aggression to your dog?
Are you nervous around big dogs?
Has your dog been around any other dogs, big or small?
The situation and your dog's fear in the class could be linked to any of the issues above. If you're nervous with big dogs, your cavvie can pick up on your anxiety and manifest its own. If an incident has occurred with one of the other dogs, that could have scared it, too. Perhaps it isnt the size of the dog that is making Maggie nervous; it could simply be that they are dogs. If she hasnt had a lot of socialization, she could simply be shy of ANY dog, big or little.
Talking to the trainer is definitely the place to start.
On an aside, that seems pricey to me for the classes. Our classes are 55 dollars, with a ten dollar discount to puppy kindergarten graduates!
Cedar (tri), Willow (blen), Holly (ruby), & Bella (blen)